Bug 1059306 - Return a dict not None if no valid search terms found; r=mdoglio

The UI expects bugs to be a dict, rather than None. This change also
makes the return value consistent with that when a search term is
identified, but the get_bugs_for_search_term() found zero bugs.
Merge pull request #193 from mozilla/1059306-bugs-none

Bug 1059306 - Return a dict not None if no valid search terms found; r=mdoglio