MBS-13538: Removing all rows in the artist credit editor makes it disappear

[mwiencek]

Problem

See the description / animated GIF in MBS-13538.

Solution

Moving the firstNameAutocomplete assignment to prevent executing newNames[0].artist on an array of length 0 is sufficient to fix the bug. However, I also made it so that you can't remove the last remaining (non-removed) AC name row; instead, clicking the remove button will simply clear it.

Testing

Tested manually, plus added a Selenium test.