Fix pushdown of degenerate HAVING clauses

Fix pushdown of degenerate HAVING clauses

67a54b9e8 taught the planner to push down HAVING clauses even when
grouping sets are present, as long as the clause does not reference
any columns that are nullable by the grouping sets.  However, there
was an oversight: if any empty grouping sets are present, the
aggregation node can produce a row that did not come from the input,
and pushing down a HAVING clause in this case may cause us to fail to
filter out that row.

Currently, non-degenerate HAVING clauses are not pushed down when
empty grouping sets are present, since the empty grouping sets would
nullify the vars they reference.  However, degenerate (variable-free)
HAVING clauses are not subject to this restriction and may be
incorrectly pushed down.

To fix, explicitly check for the presence of empty grouping sets and
retain degenerate clauses in HAVING when they are present.  This
ensures that we don't emit a bogus aggregated row.  A copy of each
such clause is also put in WHERE so that query_planner() can use it in
a gating Result node.

To facilitate this check, this patch expands the groupingSets tree of
the query to a flat list of grouping sets before applying the HAVING
pushdown optimization.  This does not add any additional planning
overhead, since we need to do this expansion anyway.

In passing, make a small tweak to preprocess_grouping_sets() by
reordering its initial operations a bit.

Backpatch to v18, where this issue was introduced.

Reported-by: Yuhang Qiu <iamqyh@gmail.com>
Author: Richard Guo <guofenglinux@gmail.com>
Author: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://postgr.es/m/0879D9C9-7FE2-4A20-9593-B23F7A0B5290@gmail.com
Backpatch-through: 18