previously the query string queries were modified to aid in
compatibility with other search systems. this change:
f391b991c2
has a problem when combined with:
77101ae424
due to the introduction of MatchNoneSearchers being returned
in a case where previously they never would.
the fix for now is to simply return disjunction queries on 0
terms instead. this ultimately also matches nothing, but avoids
triggering the logic which handles match none searchers in a
special way.
4c801f2f01