Fix a couple of planner bugs introduced by the new ability to discard

Fix a couple of planner bugs introduced by the new ability to discard
ORDER BY <constant> as redundant.  One is that this means query_planner()
has to canonicalize pathkeys even when the query jointree is empty;
the canonicalization was always a no-op in such cases before, but no more.
Also, we have to guard against thinking that a set-returning function is
"constant" for this purpose.  Add a couple of regression tests for these
evidently under-tested cases.  Per report from Greg Stark and subsequent
experimentation.