DocTypeGridEditorHelper - added null reference check on the JSON data

I encountered an edge-case where a DTGE grid cell was created without any JSON data,
storing the value as `null`. This was passed through to the `ConvertValueToContent` method,
causing the `JsonConvert` deserializer to throw a null-reference exception.

Author: leekelleher

src/Our.Umbraco.DocTypeGridEditor/Helpers/DocTypeGridEditorHelper.cs

@@ -28,6 +28,9 @@ public static IPublishedContent ConvertValueToContent(string id, string contentT
             if (string.IsNullOrWhiteSpace(contentTypeAlias))
                 return null;
 
+            if (dataJson == null)
+                return null;
+
             if (UmbracoContext.Current == null)
                 return ConvertValue(id, contentTypeAlias, dataJson);