Repository: goadesign/goa (go)
Issue: Goa panics when a user provides an explicit `null` where a `Map` is expected, if it's a nested structure
**Repro steps**

design
```go
Method("Create", func() {
	Payload(func() {
		Attribute("foo", MapOf(String, TestStruct))
		Required("foo")
	})

	HTTP(func() {
		POST("/")
		Response(StatusCreated)
	})
})


var TestStruct = CommonType("TestStruct", func() {
	Attribute("hello", TestStructTwo)
})

var TestStructTwo = CommonType("TestStructTwo", func() {
	Attribute("again", String)
})

```

payload
```json
{"foo": {"bar": null } }
```

**Expected behaviour**

Some kind of human readable error, like `struct at `foo.bar` cannot be null


**Resulting error:**

```
             	/usr/local/go/src/runtime/panic.go:770 +0x124
             github.com/incident-io/core/server/api/public/gen/http/catalog_v2/server.unmarshalTestStructRequestBodyToCommonTestStruct(...)
             	/Users/lcurtis/repos/core/server/api/public/gen/http/catalog_v2/server/encode_decode.go:524
             github.com/incident-io/core/server/api/public/gen/http/catalog_v2/server.NewCreateEntryPayload(0x14004b19db0)
             	/Users/lcurtis/repos/core/server/api/public/gen/http/catalog_v2/server/types.go:590 +0x3a8
             github.com/incident-io/core/server/api/public/gen/http/catalog_v2/server.NewCreateEntryHandler.DecodeCreateEntryRequest.func2(0x14005218d80)
             	/Users/lcurtis/repos/core/server/api/public/gen/http/catalog_v2/server/encode_decode.go:256 +0x1d8
             github.com/incident-io/core/server/api/public/gen/http/catalog_v2/server.NewCreateEntryHandler.func1({0x10d9c85b0, 0x14001468a80}, 0x14005218d80)
             	/Users/lcurtis/repos/core/server/api/public/gen/http/catalog_v2/server/server.go:508 +0x1a4
             net/http.HandlerFunc.ServeHTTP(0x1400539db90, {0x10d9c85b0, 0x14001468a80}, 0x14005218d80)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/go-chi/chi/v5.(*Mux).routeHTTP(0x14004d79aa0, {0x10d9c85b0, 0x14001468a80}, 0x14005218d80)
             	/Users/lcurtis/go/pkg/mod/github.com/go-chi/chi/v5@v5.0.11/mux.go:443 +0x2e0
             net/http.HandlerFunc.ServeHTTP(0x1400538e5d0, {0x10d9c85b0, 0x14001468a80}, 0x14005218d80)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/go-chi/chi/v5.(*Mux).ServeHTTP(0x14004d79aa0, {0x10d9c85b0, 0x14001468a80}, 0x14005218d80)
             	/Users/lcurtis/go/pkg/mod/github.com/go-chi/chi/v5@v5.0.11/mux.go:90 +0x5d4
             github.com/incident-io/core/server/api.BuildHTTP.func6.AuthenticationSourceProvider.1({0x10d9c85b0, 0x14001468a80}, 0x14005218b40)
             	/Users/lcurtis/repos/core/server/api/mw/authentication.go:82 +0x18c
             net/http.HandlerFunc.ServeHTTP(0x1400544bd10, {0x10d9c85b0, 0x14001468a80}, 0x14005218b40)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/incident-io/core/server/cmd/app/cmd.Run.func61.Run.func61.ObserveZendeskHTTP.1.2({0x10d9c85b0, 0x14001468a80}, 0x14005218b40)
             	/Users/lcurtis/repos/core/server/api/mw/observe.go:196 +0x98
             net/http.HandlerFunc.ServeHTTP(0x14005458750, {0x10d9c85b0, 0x14001468a80}, 0x14005218b40)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/incident-io/core/server/cmd/app/cmd.Run.func62.Run.func62.ObserveMobileHTTP.1.2({0x10d9c85b0, 0x14001468a80}, 0x14005218b40)
             	/Users/lcurtis/repos/core/server/api/mw/observe.go:174 +0xc8
             net/http.HandlerFunc.ServeHTTP(0x1400544bd28, {0x10d9c85b0, 0x14001468a80}, 0x14005218b40)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/incident-io/core/server/cmd/app/cmd.Run.func63.AuthenticationFromStore.1({0x10d9c85b0, 0x14001468a80}, 0x14005218a20)
             	/Users/lcurtis/repos/core/server/api/mw/authentication.go:133 +0x124
             net/http.HandlerFunc.ServeHTTP(0x1400544bd40, {0x10d9c85b0, 0x14001468a80}, 0x14005218a20)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/incident-io/core/server/cmd/app/cmd.Run.func64.Run.func64.WithSession.1.2({0x10d9c85b0, 0x14001468a80}, 0x140052187e0)
             	/Users/lcurtis/repos/core/server/api/mw/sessions.go:283 +0x3e0
             net/http.HandlerFunc.ServeHTTP(0x14005458780, {0x10d9c85b0, 0x14001468a80}, 0x140052187e0)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/incident-io/core/server/cmd/app/cmd.Run.func65.Run.func65.AuthenticationFromToken.1.2({0x10d9c85b0, 0x14001468a80}, 0x14005041d40)
             	/Users/lcurtis/repos/core/server/api/mw/authentication.go:105 +0x224
             net/http.HandlerFunc.ServeHTTP(0x140054587b0, {0x10d9c85b0, 0x14001468a80}, 0x14005041d40)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/incident-io/core/server/api/mw.SinglePageApp.func1.1({0x10d9c85b0, 0x14001468a80}, 0x14005041d40)
             	/Users/lcurtis/repos/core/server/api/mw/single_page_app.go:23 +0xb0
             net/http.HandlerFunc.ServeHTTP(0x14005387860, {0x10d9c85b0, 0x14001468a80}, 0x14005041d40)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/incident-io/core/server/cmd/app/cmd.Run.func67.APIRewrite.1({0x10d9c85b0, 0x14001468a80}, 0x14005041d40)
             	/Users/lcurtis/repos/core/server/api/mw/api_rewrite.go:33 +0x334
             net/http.HandlerFunc.ServeHTTP(0x1400544bd58, {0x10d9c85b0, 0x14001468a80}, 0x14005041d40)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/incident-io/core/server/api/mw.ApplySecurity.(*Secure).Handler.func1({0x10d9c85b0, 0x14001468a80}, 0x14005041d40)
             	/Users/lcurtis/go/pkg/mod/github.com/unrolled/secure@v1.13.0/secure.go:203 +0xf8
             net/http.HandlerFunc.ServeHTTP(0x1400544eba0, {0x10d9c85b0, 0x14001468a80}, 0x14005041d40)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/NYTimes/gziphandler.GzipHandlerWithOpts.func1.1({0x10d9d35f0, 0x1400161ea80}, 0x14005041d40)
             	/Users/lcurtis/go/pkg/mod/github.com/!n!y!times/gziphandler@v1.1.1/gzip.go:338 +0x424
             net/http.HandlerFunc.ServeHTTP(0x14005458930, {0x10d9d35f0, 0x1400161ea80}, 0x14005041d40)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/incident-io/core/server/cmd/app/cmd.Run.func70.CacheResponses.1({0x10d9d35f0, 0x1400161ea80}, 0x14005041d40)
             	/Users/lcurtis/repos/core/server/api/mw/cache_responses.go:58 +0x1a4
             net/http.HandlerFunc.ServeHTTP(0x14005454ac0, {0x10d9d35f0, 0x1400161ea80}, 0x14005041d40)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/incident-io/core/server/cmd/app/cmd.Run.func71.NoCache.1({0x10d9d35f0, 0x1400161ea80}, 0x14005041d40)
             	/Users/lcurtis/repos/core/server/api/mw/no_cache.go:25 +0x10c
             net/http.HandlerFunc.ServeHTTP(0x1400544bd70, {0x10d9d35f0, 0x1400161ea80}, 0x14005041d40)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             go.opencensus.io/plugin/ochttp.(*Handler).ServeHTTP(0x14005387950, {0x10d9d35f0, 0x1400161ea80}, 0x14005041d40)
             	/Users/lcurtis/go/pkg/mod/go.opencensus.io@v0.24.0/plugin/ochttp/server.go:92 +0x39c
             github.com/incident-io/core/server/api/mw.observeHTTP.func1.2({0x10d9c83a0, 0x14004c750a0}, 0x140050419e0)
             	/Users/lcurtis/repos/core/server/api/mw/observe.go:326 +0x1424
             net/http.HandlerFunc.ServeHTTP(0x14005458960, {0x10d9c83a0, 0x14004c750a0}, 0x140050410e0)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             github.com/getsentry/sentry-go/http.(*Handler).Handle.(*Handler).handle.func1({0x10d9c83a0, 0x14004c750a0}, 0x140050410e0)
             	/Users/lcurtis/go/pkg/mod/github.com/incident-io/sentry-go@v0.0.0-20230906150740-4d409645eb55/http/sentryhttp.go:116 +0x4bc
             net/http.HandlerFunc.ServeHTTP(0x1400544ebc0, {0x10d9c83a0, 0x14004c750a0}, 0x14005040a20)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             goa.design/goa/v3/http/middleware.RequestID.func1.1({0x10d9c83a0, 0x14004c750a0}, 0x14005040900)
             	/Users/lcurtis/go/pkg/mod/goa.design/goa/v3@v3.14.6/http/middleware/requestid.go:43 +0x29c
             net/http.HandlerFunc.ServeHTTP(0x14005454b00, {0x10d9c83a0, 0x14004c750a0}, 0x14005040900)
             	/usr/local/go/src/net/http/server.go:2166 +0x40
             net/http.serverHandler.ServeHTTP({0x14005212960}, {0x10d9c83a0, 0x14004c750a0}, 0x14005040900)
             	/usr/local/go/src/net/http/server.go:3137 +0x2a0
             net/http.(*conn).serve(0x140055463f0, {0x10d9dce40, 0x140052e0a50})
             	/usr/local/go/src/net/http/server.go:2039 +0x1180
             created by net/http.(*Server).Serve in goroutine 420
             	/usr/local/go/src/net/http/server.go:3285 +0xa84
```



Pull request description: Properly handle decoding of maps with nil entries
Fix #3483 

Patch:
diff --git a/codegen/go_transform.go b/codegen/go_transform.go
index fc29ce5b..805ea43e 100644
--- a/codegen/go_transform.go
+++ b/codegen/go_transform.go
@@ -72,24 +72,6 @@ func GoTransform(source, target *expr.AttributeExpr, sourceVar, targetVar string
 	return strings.TrimRight(code, "\n"), funcs, nil
 }
 
-// transformPrimitive returns the code to transform source primtive type to
-// target primitive type. It returns an error if source and target are not
-// compatible for transformation.
-func transformPrimitive(source, target *expr.AttributeExpr, sourceVar, targetVar string, newVar bool, ta *TransformAttrs) (string, error) {
-	if err := IsCompatible(source.Type, target.Type, sourceVar, targetVar); err != nil {
-		return "", err
-	}
-	assign := "="
-	if newVar {
-		assign = ":="
-	}
-	if source.Type.Name() != target.Type.Name() {
-		cast := ta.TargetCtx.Scope.Ref(target, ta.TargetCtx.Pkg(target))
-		return fmt.Sprintf("%s %s %s(%s)\n", targetVar, assign, cast, sourceVar), nil
-	}
-	return fmt.Sprintf("%s %s %s\n", targetVar, assign, sourceVar), nil
-}
-
 // transformAttribute returns the code to transform source attribute to target
 // attribute. It returns an error if source and target are not compatible for
 // transformation.
@@ -112,6 +94,24 @@ func transformAttribute(source, target *expr.AttributeExpr, sourceVar, targetVar
 	return
 }
 
+// transformPrimitive returns the code to transform source primtive type to
+// target primitive type. It returns an error if source and target are not
+// compatible for transformation.
+func transformPrimitive(source, target *expr.AttributeExpr, sourceVar, targetVar string, newVar bool, ta *TransformAttrs) (string, error) {
+	if err := IsCompatible(source.Type, target.Type, sourceVar, targetVar); err != nil {
+		return "", err
+	}
+	assign := "="
+	if newVar {
+		assign = ":="
+	}
+	if source.Type.Name() != target.Type.Name() {
+		cast := ta.TargetCtx.Scope.Ref(target, ta.TargetCtx.Pkg(target))
+		return fmt.Sprintf("%s %s %s(%s)\n", targetVar, assign, cast, sourceVar), nil
+	}
+	return fmt.Sprintf("%s %s %s\n", targetVar, assign, sourceVar), nil
+}
+
 // transformObject generates Go code to transform source object to target
 // object.
 func transformObject(source, target *expr.AttributeExpr, sourceVar, targetVar string, newVar bool, ta *TransformAttrs) (string, error) {
@@ -682,6 +682,10 @@ for key, val := range {{ .SourceVar }} {
   {{ transformAttribute .SourceKey .TargetKey "key" "tk" true .TransformAttrs -}}
 {{ end -}}
 {{ if .IsElemStruct -}}
+	if val == nil {
+		{{ .TargetVar }}[tk] = nil
+		continue
+	}
 	{{ .TargetVar }}[tk] = {{ transformHelperName .SourceElem .TargetElem .TransformAttrs -}}(val)
 {{ else -}}
 	{{ transformAttribute .SourceElem .TargetElem "val" (printf "tv%s" .LoopVar) true .TransformAttrs -}}


Test patch:
diff --git a/codegen/go_transform_test.go b/codegen/go_transform_test.go
index 316bcb89..9ad3c268 100644
--- a/codegen/go_transform_test.go
+++ b/codegen/go_transform_test.go
@@ -432,6 +432,10 @@ const (
 		target.TypeMap = make(map[string]*SimpleMap, len(source.TypeMap))
 		for key, val := range source.TypeMap {
 			tk := key
+			if val == nil {
+				target.TypeMap[tk] = nil
+				continue
+			}
 			target.TypeMap[tk] = transformSimpleMapToSimpleMap(val)
 		}
 	}
@@ -613,6 +617,10 @@ const (
 		target.Recursive = make(map[string]*RecursiveMap, len(source.Recursive))
 		for key, val := range source.Recursive {
 			tk := key
+			if val == nil {
+				target.Recursive[tk] = nil
+				continue
+			}
 			target.Recursive[tk] = transformRecursiveMapToRecursiveMap(val)
 		}
 	}
diff --git a/http/codegen/testdata/parse_endpoint_functions.go b/http/codegen/testdata/parse_endpoint_functions.go
index 0a5c6c0c..4b5ff942 100644
--- a/http/codegen/testdata/parse_endpoint_functions.go
+++ b/http/codegen/testdata/parse_endpoint_functions.go
@@ -992,6 +992,10 @@ func BuildMethodBodyInlineMapUserPayload(serviceBodyInlineMapUserMethodBodyInlin
 	v := make(map[*servicebodyinlinemapuser.KeyType]*servicebodyinlinemapuser.ElemType, len(body))
 	for key, val := range body {
 		tk := marshalKeyTypeRequestBodyToServicebodyinlinemapuserKeyType(val)
+		if val == nil {
+			v[tk] = nil
+			continue
+		}
 		v[tk] = marshalElemTypeRequestBodyToServicebodyinlinemapuserElemType(val)
 	}
 	return v, nil
diff --git a/http/codegen/testdata/payload_constructor_functions.go b/http/codegen/testdata/payload_constructor_functions.go
index e26b5512..57b700e6 100644
--- a/http/codegen/testdata/payload_constructor_functions.go
+++ b/http/codegen/testdata/payload_constructor_functions.go
@@ -1009,6 +1009,10 @@ func NewMethodBodyInlineMapUserMapKeyTypeElemType(body map[*KeyTypeRequestBody]*
 	v := make(map[*servicebodyinlinemapuser.KeyType]*servicebodyinlinemapuser.ElemType, len(body))
 	for key, val := range body {
 		tk := unmarshalKeyTypeRequestBodyToServicebodyinlinemapuserKeyType(val)
+		if val == nil {
+			v[tk] = nil
+			continue
+		}
 		v[tk] = unmarshalElemTypeRequestBodyToServicebodyinlinemapuserElemType(val)
 	}
 	return v