Listenbrainz scrobbles are submitted with `None` if mbid is missing
I set up Listenbrainz scrobbling (yay!) and realized that at some point missing
mbids on tracks get converted to the string
None, so the scrobbled are submitted with a wrong
mbid instead of just leaving it out. This shows for example in tracks at https://listenbrainz.org/user/$user all linking to https://musicbrainz.org/recording/None. It seems there's no validation in the Listenbrainz API and for some reason
None is stringified somewhere.