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.
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