Some smaller fixes.

- Limit config core.restore_state to a known set of values.
- Initialize new_state to None instead of ''

mopidy/config/__init__.py

@@ -21,7 +21,8 @@ _core_schema['config_dir'] = Path()
 _core_schema['data_dir'] = Path()
 # MPD supports at most 10k tracks, some clients segfault when this is exceeded.
 _core_schema['max_tracklist_length'] = Integer(minimum=1, maximum=10000)
-_core_schema['restore_state'] = String(optional=True)
+_core_schema['restore_state'] = String(
+    optional=True, choices=['off', 'volume', 'load', 'last', 'play'])
 
 _logging_schema = ConfigSchema('logging')
 _logging_schema['color'] = Boolean()

mopidy/core/playback.py

@@ -554,7 +554,7 @@ class PlaybackController(object):
         if state:
             if not isinstance(state, models.PlaybackState):
                 raise TypeError('Expect an argument of type "PlaybackState"')
-            new_state = ''
+            new_state = None
             if 'play-always' in coverage:
                 new_state = PlaybackState.PLAYING
             if 'play-last' in coverage: