MetadataParser: Don't assign thumbnails to unknown albums

Fix #272
3 jobs for !339 with dont_share_unknown_album_artist_thumbnails in 5 minutes and 6 seconds (queued for 2 seconds)
latest detached
Status Job ID Name Coverage
  Test
passed #467074
amd64 docker
linux

00:04:43

passed #467075
amd64 docker
win32

00:05:06

passed #467076
amd64 docker
win64

00:04:12