postgres_fdw: don't push ORDER BY with no vars (bug #15352)

 postgres_fdw: don't push ORDER BY with no vars (bug #15352)
 
 Commit aa09cd242 changed a condition in find_em_expr_for_rel from
 being a bms_equal comparison of relids to bms_is_subset, in order to
 support order by clauses on foreign joins. But this also allows
 through the degenerate case of expressions with no Vars at all (and
 hence empty relids), including integer constants which will be parsed
 unexpectedly on the remote (viz. "ERROR: ORDER BY position 0 is not in
 select list" as in the bug report).
 
 Repair by adding an additional !bms_is_empty test.
 
 Backpatch through to 9.6 where the aforementioned change was made.
 
 Per bug #15352 from Maksym Boguk; analysis and patch by me.
 
 Discussion: https://postgr.es/m/153518420278.1478.14875560810251994661@wrigleys.postgresql.org