Remove query_planner's overhasty rejection of cases where

Remove query_planner's overhasty rejection of cases where
tlist and qual are NULL.  It ought to handle these the same as the cases
where tlist contains only constant expressions, ie, be willing to generate
a Result-node plan.  This did not use to matter, but it does now because
union_planner will flatten the tlist when aggregates are present.  Thus,
'select count(1) from table' now causes query_planner to be given a null
tlist, and to duplicate 6.4's behavior we need it to give back a Result
plan rather than refusing the query.  6.4 was arguably doing the Wrong
Thing for this query, but I'm not going to open a semantics issue right
before 6.5 release ... can revisit that problem later.