MBS-6918: artist credit changes break /release/add on beta

Problem 1:

I placed a copy of submitted_release_artist in the stash, but I added the relevant line to prepare_tracklist in ReleaseEditor::Edit, which obviously won't affect ReleaseEditor::Add. Now the line is in the main prepare_tracklist function. (I haven't found out how this was working locally before, because I specifically tested /release/add, but I must've changed something at the last minute.)

Problem 2:

MB.Control.ArtistCredit expected initialData to not be an empty list. I'm not sure if it's possible to be now that I've fixed problem 1, but just to be on the safe side, I've modified Form::Field::ArtistCredit and MB.Control.ArtistCredit to not return an empty list and not assume it's dealing with a non-empty list, respectively.

Problem 3:

This escaped my initial testing. If you parse a tracklist with no artists, the tracklist would no longer default to the release artist. That's now fixed.