Fix null values handling in C# Json serialization

It didn't replace prefab object reference if replaced with null in the prefab instance.

Source/Engine/Serialization/JsonSerializer.cs

@@ -73,7 +73,7 @@ namespace FlaxEngine.Json
                 ContractResolver = new ExtendedDefaultContractResolver(isManagedOnly),
                 ReferenceLoopHandling = ReferenceLoopHandling.Ignore,
                 TypeNameHandling = TypeNameHandling.Auto,
-                NullValueHandling = NullValueHandling.Ignore,
+                NullValueHandling = NullValueHandling.Include,
                 ObjectCreationHandling = ObjectCreationHandling.Auto,
             };
             if (ObjectConverter == null)
@@ -276,6 +276,11 @@ namespace FlaxEngine.Json
             cache.IsDuringSerialization = false;
             Current.Value = cache;
 
+            /*// Debug json string reading
+            cache.MemoryStream.Initialize(jsonBuffer, jsonLength);
+            cache.Reader.DiscardBufferedData();
+            string json = cache.Reader.ReadToEnd();*/
+
             cache.MemoryStream.Initialize(jsonBuffer, jsonLength);
             cache.Reader.DiscardBufferedData();
             var jsonReader = new JsonTextReader(cache.Reader);