catch cook-id urls with no ids supplied even if there is no final second slash. Don't make requests for such lists if the ids list is empty.

I've been getting an error with debug=False using nginx and uWSGI with empty cooked-id fields. At some point (not sure where) the final slash in a url like ..../cook-ids/field_name// is trimmed off - and then the request falls through to normal django admin which raises a 500 because cook_ids... is not a valid int. This commit ensures that those requests don't fall through to normal admin, and furthermore updates the javascript so empty requests aren't made in any case.