allow JsonGuidConverter to read null

MediaBrowser.Common/Json/Converters/JsonGuidConverter.cs

@@ -11,7 +11,11 @@ namespace MediaBrowser.Common.Json.Converters
     {
         /// <inheritdoc />
         public override Guid Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
-            => new Guid(reader.GetString());
+        {
+            var guidStr = reader.GetString();
+
+            return guidStr == null ? Guid.Empty : new Guid(guidStr);
+        }
 
         /// <inheritdoc />
         public override void Write(Utf8JsonWriter writer, Guid value, JsonSerializerOptions options)