pgsql: Partially undo commit 94da73281.

Partially undo commit 94da73281.

On closer inspection, mcv.c isn't as broken for ScalarArrayOpExpr
as I thought.  The Var-on-right issue is real enough, but actually
it does cope fine with a NULL array constant --- I was misled by
an XXX comment suggesting it didn't.  Undo that part of the code
change, and replace the XXX comment with something less misleading.

Branch
------
REL_14_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/b9243cadad55e296e83839fb96457f64dcaaeb25

Modified Files
--------------
src/backend/statistics/extended_stats.c |  7 +++----
src/backend/statistics/mcv.c            | 25 ++++++++++++++++---------
2 files changed, 19 insertions(+), 13 deletions(-)