gpodder.model: Fix problems with migrated databases

I'm finally migrating from gpodder 2 to 3, and when starting up gpodder3
after running the migration, I ran into the following uncaught exception
(which aborted gpodder):

TypeError: coercing to Unicode: need string or buffer, NoneType found

src/gpodder/model.py

@@ -818,7 +818,7 @@ def check_download_folder(self):
         for episode in self.get_episodes(gpodder.STATE_DOWNLOADED):
             if episode.was_downloaded():
                 filename = episode.local_filename(create=False)
-                if not os.path.exists(filename):
+                if filename is not None and not os.path.exists(filename):
                     # File has been deleted by the user - simulate a
                     # delete event (also marks the episode as deleted)
                     logger.debug('Episode deleted: %s', filename)