Artist: Use a WHEN clause in triggers
Artist: Use a WHEN clause in triggers

Instead of a WHERE clause, while we can discriminate unknown/various
artists early
There is no migration required since the Artist table gets dropped as
part of 13->14 migration, so the trigger will also be deleted and
recreated

chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary,chouquette/medialibrary