Having an operation for fallback_content_rating would be really benefitial for people who use the mass_*_content_rating operations. Currently, if there is no content rating found on the "new" system then the existing content rating is left in place, I would like to be able to set any content rating not found in the new system to "Not Rated" so that I can avoid the clutter like this (Approved, NR, Passed).

The idea being that the user can specify the text for any content that does not have a rating in the new system.

This could also be extended to fallback_genre_update