Converting a track to an actual URI might cause unhanded exceptions

[adamcik]

By the look of https://github.com/mopidy/mopidy-beets/blob/master/mopidy_beets/actor.py#L37 the code won't nicely handle an invalid track ID. This should be fixed so the code returns None in such cases.

[sumpfralle]

thank you for pointing out this issue!
Commit 471dac7 fixed this problem.