fix: added WriteUndefined as there is Newtonsoft.Json

Author: Denny09310

src/ElectronNET.API/Converter/TitleBarOverlayConverter.cs

@@ -26,6 +26,7 @@ public override void WriteJson(JsonWriter writer, TitleBarOverlay value, JsonSer
     {
         if (value is null)
         {
+            writer.WriteUndefined();
             return;
         }