Prevent crash if MusicBrainz release date missing

If MusicBrainz returns data for the CD being ripped but no release date
is returned, we attempt to set the DATE tag to None (which is caught by
mutagen).  To resolve this, check if there is a value for release date
before setting.

morituri/common/program.py

@@ -462,7 +462,8 @@ class Program:
         tags['TRACKNUMBER'] = u'%s' % number
 
         if self.metadata:
-            tags['DATE'] = self.metadata.release
+            if self.metadata.release is not None:
+                tags['DATE'] = self.metadata.release
 
             if number > 0:
                 tags['MUSICBRAINZ_TRACKID'] = mbidTrack