Skip Discogs query on insufficiently tagged files
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.

beetbox/beets,beetbox/beets,beetbox/beets,beetbox/beets