fix: prevent error when sorting an artist's tracks

...by falling back to using `0` when a sort key is `None`.
This makes the artist detail view also show up when one of the artist's tracks is missing a field that is used for sorting.

Closes #42

mopidy_beets/library.py

@@ -166,7 +166,9 @@ def lookup(self, uri=None, uris=None):
                 )
                 # Append composer tracks to the artist tracks (unique items).
                 tracks = list(set(artist_tracks + composer_tracks))
-                tracks.sort(key=lambda t: (t.date, t.disc_no, t.track_no))
+                tracks.sort(
+                    key=lambda t: (t.date or 0, t.disc_no or 0, t.track_no or 0)
+                )
             else:
                 logger.info("Unknown Beets lookup URI: %s", uri)
                 tracks = []