Skip Discogs query on insufficiently tagged files

- When files are missing both, album and artist tags, the Discogs metadata
  plugin sends empty information to the Discogs API which returns arbitrary
  query results.
- This patch catches this case and states it in beets import verbose output.

beetsplug/discogs.py

@@ -157,6 +157,11 @@ class DiscogsPlugin(BeetsPlugin):
         if not self.discogs_client:
             return
 
+        if not album and not artist:
+            self._log.debug('Skipping Discogs query. Files missing album and '
+                            'artist tags.')
+            return []
+
         if va_likely:
             query = album
         else: