Repository: goccy/go-json (go)

Issue:
Unmarshal fails when decoding `null` as slice
Unmarshal fails when decoding `null` as slice.
Referring to `encoding/json`, the expected result is typed `nil`.
This issue is similar to #5 .

## Reproduction code

```go
package main

import (
	encodingjson "encoding/json"
	"fmt"

	goccyjson "github.com/goccy/go-json"
)

type Message struct {
	Fields []string `json:"field"`
}

func main() {
	ok := `{"field": ["value"]}`
	ng := `{"field": null}`

	fmt.Println("encoding/json")
	for _, s := range []string{ok, ng} {
		var msg Message
		err := encodingjson.Unmarshal([]byte(s), &msg)
		if err != nil {
			panic(err)
		}
		fmt.Printf("%#v\n", msg)
	}

	fmt.Println("goccy/go-json")
	for _, s := range []string{ok, ng} {
		var msg Message
		err := goccyjson.Unmarshal([]byte(s), &msg)
		if err != nil {
			panic(err)
		}
		fmt.Printf("%#v\n", msg)
	}
}
```

> encoding/json
main.Message{Fields:[]string{"value"}}
main.Message{Fields:[]string(nil)}
goccy/go-json
main.Message{Fields:[]string{"value"}}
panic: unexpected end of JSON input for slice
> 
> goroutine 1 [running]:
main.main()
	/tmp/sandbox005927213/prog.go:33 +0x54d


https://play.golang.org/p/Vtr490WAF2e

Pull request description:
Fix handling of null value for some types
resolve #7 

Fix `slice` , `array` `map`  types

Patch:
diff --git a/decode_array.go b/decode_array.go
index 7e34d85..5a0eac7 100644
--- a/decode_array.go
+++ b/decode_array.go
@@ -20,6 +20,11 @@ func (d *arrayDecoder) decodeStream(s *stream, p uintptr) error {
 	for {
 		switch s.char() {
 		case ' ', '\n', '\t', '\r':
+		case 'n':
+			if err := nullBytes(s); err != nil {
+				return err
+			}
+			return nil
 		case '[':
 			idx := 0
 			for {
@@ -63,6 +68,22 @@ func (d *arrayDecoder) decode(buf []byte, cursor int64, p uintptr) (int64, error
 		switch buf[cursor] {
 		case ' ', '\n', '\t', '\r':
 			continue
+		case 'n':
+			buflen := int64(len(buf))
+			if cursor+3 >= buflen {
+				return 0, errUnexpectedEndOfJSON("null", cursor)
+			}
+			if buf[cursor+1] != 'u' {
+				return 0, errInvalidCharacter(buf[cursor+1], "null", cursor)
+			}
+			if buf[cursor+2] != 'l' {
+				return 0, errInvalidCharacter(buf[cursor+2], "null", cursor)
+			}
+			if buf[cursor+3] != 'l' {
+				return 0, errInvalidCharacter(buf[cursor+3], "null", cursor)
+			}
+			cursor += 4
+			return cursor, nil
 		case '[':
 			idx := 0
 			for {
diff --git a/decode_map.go b/decode_map.go
index 2ae99fb..cd36587 100644
--- a/decode_map.go
+++ b/decode_map.go
@@ -47,7 +47,14 @@ func (d *mapDecoder) setValueStream(s *stream, key interface{}) error {
 
 func (d *mapDecoder) decodeStream(s *stream, p uintptr) error {
 	s.skipWhiteSpace()
-	if s.char() != '{' {
+	switch s.char() {
+	case 'n':
+		if err := nullBytes(s); err != nil {
+			return err
+		}
+		return nil
+	case '{':
+	default:
 		return errExpected("{ character for map value", s.totalOffset())
 	}
 	mapValue := makemap(d.mapType, 0)
@@ -94,7 +101,24 @@ func (d *mapDecoder) decode(buf []byte, cursor int64, p uintptr) (int64, error)
 	if buflen < 2 {
 		return 0, errExpected("{} for map", cursor)
 	}
-	if buf[cursor] != '{' {
+	switch buf[cursor] {
+	case 'n':
+		if cursor+3 >= buflen {
+			return 0, errUnexpectedEndOfJSON("null", cursor)
+		}
+		if buf[cursor+1] != 'u' {
+			return 0, errInvalidCharacter(buf[cursor+1], "null", cursor)
+		}
+		if buf[cursor+2] != 'l' {
+			return 0, errInvalidCharacter(buf[cursor+2], "null", cursor)
+		}
+		if buf[cursor+3] != 'l' {
+			return 0, errInvalidCharacter(buf[cursor+3], "null", cursor)
+		}
+		cursor += 4
+		return cursor, nil
+	case '{':
+	default:
 		return 0, errExpected("{ character for map value", cursor)
 	}
 	cursor++
diff --git a/decode_slice.go b/decode_slice.go
index 34ffad8..974a6df 100644
--- a/decode_slice.go
+++ b/decode_slice.go
@@ -62,6 +62,11 @@ func (d *sliceDecoder) decodeStream(s *stream, p uintptr) error {
 		case ' ', '\n', '\t', '\r':
 			s.cursor++
 			continue
+		case 'n':
+			if err := nullBytes(s); err != nil {
+				return err
+			}
+			return nil
 		case '[':
 			idx := 0
 			slice := d.newSlice()
@@ -136,6 +141,22 @@ func (d *sliceDecoder) decode(buf []byte, cursor int64, p uintptr) (int64, error
 		switch buf[cursor] {
 		case ' ', '\n', '\t', '\r':
 			continue
+		case 'n':
+			buflen := int64(len(buf))
+			if cursor+3 >= buflen {
+				return 0, errUnexpectedEndOfJSON("null", cursor)
+			}
+			if buf[cursor+1] != 'u' {
+				return 0, errInvalidCharacter(buf[cursor+1], "null", cursor)
+			}
+			if buf[cursor+2] != 'l' {
+				return 0, errInvalidCharacter(buf[cursor+2], "null", cursor)
+			}
+			if buf[cursor+3] != 'l' {
+				return 0, errInvalidCharacter(buf[cursor+3], "null", cursor)
+			}
+			cursor += 4
+			return cursor, nil
 		case '[':
 			idx := 0
 			slice := d.newSlice()


Test patch:
diff --git a/decode_test.go b/decode_test.go
index 562f00c..61d004a 100644
--- a/decode_test.go
+++ b/decode_test.go
@@ -135,12 +135,26 @@ func Test_Decoder(t *testing.T) {
 		assertEq(t, "struct.D.AA", 2, v.D.AA)
 		assertEq(t, "struct.D.BB", "world", v.D.BB)
 		assertEq(t, "struct.D.CC", true, v.D.CC)
-		t.Run("struct.null", func(t *testing.T) {
+		t.Run("struct.field null", func(t *testing.T) {
 			var v struct {
 				A string
+				B []string
+				C []int
+				D map[string]interface{}
+				E [2]string
+				F interface{}
 			}
-			assertErr(t, json.Unmarshal([]byte(`{"a":null}`), &v))
-			assertEq(t, "string is null", v.A, "")
+			assertErr(t, json.Unmarshal([]byte(`{"a":null,"b":null,"c":null,"d":null,"e":null,"f":null}`), &v))
+			assertEq(t, "string", v.A, "")
+			assertNeq(t, "[]string", v.B, nil)
+			assertEq(t, "[]string", len(v.B), 0)
+			assertNeq(t, "[]int", v.C, nil)
+			assertEq(t, "[]int", len(v.C), 0)
+			assertNeq(t, "map", v.D, nil)
+			assertEq(t, "map", len(v.D), 0)
+			assertNeq(t, "array", v.E, nil)
+			assertEq(t, "array", len(v.E), 2)
+			assertEq(t, "interface{}", v.F, nil)
 		})
 	})
 	t.Run("interface", func(t *testing.T) {
diff --git a/helper_test.go b/helper_test.go
index 26c0f12..53157ce 100644
--- a/helper_test.go
+++ b/helper_test.go
@@ -15,3 +15,10 @@ func assertEq(t *testing.T, msg string, exp interface{}, act interface{}) {
 		t.Fatalf("failed to test for %s. exp=[%v] but act=[%v]", msg, exp, act)
 	}
 }
+
+func assertNeq(t *testing.T, msg string, exp interface{}, act interface{}) {
+	t.Helper()
+	if exp == act {
+		t.Fatalf("failed to test for %s. expected value is not [%v] but got same value", msg, act)
+	}
+}