Looks like Plex has implemented this as part of the newly released release of Plex Media Server version 1.20.0 (currently only available in the beta channel).
Items with no user rating will be listed at the end of the library when sorted this way (e.g. if a movie only has an IMDB but no rotten tomatoes rating).
Edit:
Closing this request in order to return each user’s votes so you can upvote more suggestions
