embunit: fix macros

```diff
@@ -46,9 +46,9 @@ void assertImplementationCStr(const char *expected,const char *actual, long line
 
 #define TEST_ASSERT_EQUAL_STRING(expected_, actual_) \
     do { \
-        __typeof__(expected_) ____expected__ = (expected_); \
-        __typeof__(actual_) ____actual__ = (actual_); \
-        if ((!____expected__ != !____actual__) || (____expected__ && stdimpl_strcmp(____expected__, ____actual__) != 0)
+        __typeof__(expected_) ____expected__ = expected_; \
+        __typeof__(actual_) ____actual__ = actual_; \
+        if (____expected__ == NULL || ____actual__ == NULL || stdimpl_strcmp(____expected__, ____actual__) != 0) { \
             assertImplementationCStr(____expected__, ____actual__, __LINE__, __FILE__); \
             return; \
         } \
```