Extend the existing support for querying blank parameters in a search.

For example (but not limited to):

Query: iswc=/-
Query: isrc=/-

Allow these searched to be combined with other parameters, as with a standard advanced query syntax search:

Query: artist:"Foobar" AND iswc:/-

(Note: replace the forward-slashes with back-slashes, the ticket won't display a back-slash)