Fix regression in rapidjson update

Source/Engine/Serialization/JsonWriter.h

@@ -86,7 +86,7 @@ public:
 
     void String(const StringAnsi& value)
     {
-        String(value.Get(), static_cast<unsigned>(value.Length()));
+        String(value.Get(), value.Length());
     }
 
     FORCE_INLINE void RawValue(const StringAnsi& str)

Source/ThirdParty/rapidjson/writer.h

@@ -202,7 +202,6 @@ public:
     }
 
     bool String(const Ch* str, SizeType length, bool copy = false) {
-        RAPIDJSON_ASSERT(str != 0);
         (void)copy;
         Prefix(kStringType);
         return EndValue(WriteString(str, length));