Fix the scrobbler plugin submitting literal "None" as MusicBrainz ID

The scrobbler plugin would submit the string value "None" if 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)