Album artist is an optional field. Fix for #15.
Album artist is an optional field. Fix for #15.

jaapz/mopidy-gmusic,elrosti/mopidy-gmusic,jaibot/mopidy-gmusic,jeh/mopidy-gmusic,hechtus/mopidy-gmusic,Tilley/mopidy-gmusic,jodal/mopidy-gmusic,mopidy/mopidy-gmusic