The scrobbler plugin would submit the string value
track.mbid is empty.
According to last.fm scrobber 1.2 docs (https://web.archive.org/web/20090217162831/http://last.fm/api/submissions) it should be:
The MusicBrainz Track ID, or an empty string if not known.
According to the 2.0 scrobble API https://www.last.fm/api/show/track.scrobble it is
mbid[i] (Optional) : The MusicBrainz Track ID.
That indicates an empty value is ok. But in any case
"None" is not :)
Fixes #1498 (closed)
Closes #1498 (closed)