Avoid generating empty query terms
Avoid generating empty query terms

If the list of values for a term is empty, skip generation
of the query string for that term rather than emitting ()

Signed-off-by: Daniel P. Berrange <bb938cf255e055ff3507f2627d214e8e62118fcf@redhat.com>

berrange/gerrymander,Dacelonid/gerrymander,agx/gerrymander,berrange/gerrymander,Dacelonid/gerrymander,agx/gerrymander