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);

Source/Engine/UI/GUI/Control.cs

@@ -877,7 +877,7 @@ namespace FlaxEngine.GUI
         /// <summary>
         /// Gets or sets the tooltip text.
         /// </summary>
-        [HideInEditor]
+        [HideInEditor, NoSerialize]
         public string TooltipText
         {
             get => _tooltipText;
@@ -887,7 +887,7 @@ namespace FlaxEngine.GUI
         /// <summary>
         /// Gets or sets the custom tooltip control linked. Use null to show default shared tooltip from the current <see cref="Style"/>.
         /// </summary>
-        [HideInEditor]
+        [HideInEditor, NoSerialize]
         public Tooltip CustomTooltip
         {
             get => _tooltip;

Source/Tools/FlaxEngine.Tests/TestSerialization.cs

@@ -47,15 +47,15 @@ namespace FlaxEngine.Tests
         {
             ObjectOne obj = new ObjectOne();
 
-            Assert.AreEqual("{\n\t\"MyValue\": 0.0,\n\t\"MyVector\": {\n\t\t\"X\": 0.0,\n\t\t\"Y\": 0.0\n\t}\n}", FilterLineBreak(JsonSerializer.Serialize(obj)));
+            Assert.AreEqual("{\n\t\"MyValue\": 0.0,\n\t\"MyVector\": {\n\t\t\"X\": 0.0,\n\t\t\"Y\": 0.0\n\t},\n\t\"MyArray\": null\n}", FilterLineBreak(JsonSerializer.Serialize(obj)));
 
             obj.MyValue = 1.2f;
 
-            Assert.AreEqual("{\n\t\"MyValue\": 1.2,\n\t\"MyVector\": {\n\t\t\"X\": 0.0,\n\t\t\"Y\": 0.0\n\t}\n}", FilterLineBreak(JsonSerializer.Serialize(obj)));
+            Assert.AreEqual("{\n\t\"MyValue\": 1.2,\n\t\"MyVector\": {\n\t\t\"X\": 0.0,\n\t\t\"Y\": 0.0\n\t},\n\t\"MyArray\": null\n}", FilterLineBreak(JsonSerializer.Serialize(obj)));
 
             obj.MyVector.Y = 2.0f;
 
-            Assert.AreEqual("{\n\t\"MyValue\": 1.2,\n\t\"MyVector\": {\n\t\t\"X\": 0.0,\n\t\t\"Y\": 2.0\n\t}\n}", FilterLineBreak(JsonSerializer.Serialize(obj)));
+            Assert.AreEqual("{\n\t\"MyValue\": 1.2,\n\t\"MyVector\": {\n\t\t\"X\": 0.0,\n\t\t\"Y\": 2.0\n\t},\n\t\"MyArray\": null\n}", FilterLineBreak(JsonSerializer.Serialize(obj)));
 
             obj.MyArray = new[]
             {