The ckan search interface exposes it's SOLR backend so most of those docs are useful.
I haven't managed to find a way to list facets but once you've guessed the name of one:
http://catalog.data.gov/api/3/action/package_search?facet.field=license_id&fq=license_id:other-license-specified
or if the value has spaces in it you can quote the search license_title:"Creative Commons Attribution"
:
http://catalog.data.gov/api/3/action/package_search?facet.field=license_title&fq=license_title:%22Creative%20Commons%20Attribution%22
if you're looking for a nil value and not the empty string then excluding the range -field:[* TO *]
:
http://catalog.data.gov/api/3/action/package_search?facet.field=license_id&fq=-license_id:[*%20TO%20*]