Propagate query-arg-only URL to psycopg2; don't send blank host

Propagate query-arg-only URL to psycopg2; don't send blank host

Fixed regression from release 1.3.2 caused by :ticket:`4562` where a URL
that contained only a query string and no hostname, such as for the
purposes of specifying a service file with connection information, would no
longer be propagated to psycopg2 properly.   The change in :ticket:`4562`
has been adjusted to further suit psycopg2's exact requirements, which is
that if there are any connection parameters whatsoever, the "dsn" parameter
is no longer required, so in this case the query string parameters are
passed alone.

Fixes: #4601
Change-Id: Ic29a8b77bcf50ee996968bab25aaac7ae4bfc26f