audio: Make sure tags are never none (Fixes #1449)

docs/changelog.rst

@@ -33,6 +33,8 @@ Bug fix release.
 - Audio: Make sure scanner handles streams without a duration.
   (Fixes: :issue:`1526`)
 
+- Audio: Ensure audio tags are never `None`. (Fixes: :issue:`1449`)
+
 - Core: Avoid endless loop if all tracks in the tracklist are unplayable and
   consume mode is off. (Fixes: :issue:`1221`, :issue:`1454`, PR: :issue:`1455`)
 

mopidy/audio/actor.py

@@ -368,7 +368,7 @@ class _Handler(object):
 
         # Emit any postponed tags that we got after about-to-finish.
         tags, self._audio._pending_tags = self._audio._pending_tags, None
-        self._audio._tags = tags
+        self._audio._tags = tags or {}
 
         if tags:
             logger.debug('Audio event: tags_changed(tags=%r)', tags.keys())