[beets] Add a rule to match untitled albums

I’m looking at you, ( ) by Sigur Rós

beets.config.yaml

@@ -39,6 +39,7 @@ replace:
   '[-_]+':         _
   '^[-_](?!$)':    ''
   '(?<!^)[-_]$':   ''
+  '^_*$':          'Untitled'
 
 # first (non-pregap) track on each disc always has track number 1
 per_disc_numbering: yes