db/Count: include cleanup
tag/Set: do AlbumArtist/Artist fallback only if AlbumArtist is not disabled

On "list albumartist", songs that have no AlbumArtist tag will use the
Artist tag.  However, if AlbumArtist is disabled via
"metadata_to_use", the TagBuilder::AddItem() call is ignored, and
PrintUniqueTag() attempts to print a nullptr string.

This commit fixes the problem by attempting the fallback only if
AlbumArtist is not disabled.