Don't assign cover for unknown artist unknown album

We currently will share any thumbnail we find for a media that belongs to the unknown artist unknown album. This can lead to the same thumbnail being overshared in case the user's library isn't correctly tagged.

It is fine to assign the thumbnail to the media, but we shouldn't propagate it to the album & artist.

For known artists unknown album, it's OK to propagate the thumbnail to the artist, but it might not be a good idea to use it for its unknown album