Allow empty album and/or artist

I was getting errors like:

ERROR    Unhandled exception in GMusicBackend (urn:uuid:[filtered]):
Traceback (most recent call last):
  File "/usr/lib/python2.7/dist-packages/pykka/actor.py", line 191, in _actor_loop
    self.on_start()
  File "/usr/local/lib/python2.7/dist-packages/mopidy_gmusic/actor.py", line 30, in on_start
    self.library.refresh()
  File "/usr/local/lib/python2.7/dist-packages/mopidy_gmusic/library.py", line 124, in refresh
    self._to_mopidy_track(song)
  File "/usr/local/lib/python2.7/dist-packages/mopidy_gmusic/library.py", line 207, in _to_mopidy_track
    artists=[self._to_mopidy_artist(song)],
  File "/usr/local/lib/python2.7/dist-packages/mopidy_gmusic/library.py", line 233, in _to_mopidy_artist
    name = song['artist']
KeyError: u'artist'