set current_track to None on clear commands - required by Sonata

mopidy/backends/__init__.py

@@ -87,6 +87,7 @@ class BaseCurrentPlaylistController(object):
     def clear(self):
         """Clear the current playlist."""
         self.backend.playback.stop()
+        self.backend.playback.current_track = None
         self.playlist = Playlist()
 
     def get_by_id(self, id):

tests/mpd/frontend_test.py

@@ -543,6 +543,7 @@ class CurrentPlaylistHandlerTest(unittest.TestCase):
         self.assertEquals(self.b.current_playlist.playlist.length, 5)
         result = self.h.handle_request(u'clear')
         self.assertEquals(self.b.current_playlist.playlist.length, 0)
+        self.assertEquals(self.b.playback.current_track, None)
         self.assert_(u'OK' in result)
 
     def test_delete_songpos(self):