Bug: var_is_nonnullable() gives wrong results for old/new in RETURNING

Hi hackers,

It appears the optimizer incorrectly simplifies old.<col> IS NULL to FALSE
in RETURNING clauses when the underlying column has a NOT NULL constraint.

The issue is that var_is_nonnullable() in clauses.c doesn't check
Var.varreturningtype. It sees a NOT NULL column and concludes the Var can
never be NULL.
But this assumption is wrong for old.* and new.* references. Because the
old tuple doesn't exist on INSERT, and the new tuple doesn't exist on
DELETE
I am not super familiar with this area, so I attempted to fix this as in
the patch attached.

Repro:

postgres=# CREATE TABLE t (id INT NOT NULL PRIMARY KEY, val INT);
INSERT INTO t VALUES (1, 10);

MERGE INTO t
USING (VALUES (1, 99), (2, 50)) AS s(id, val) ON t.id = s.id
WHEN MATCHED THEN UPDATE SET val = s.val
WHEN NOT MATCHED THEN INSERT VALUES (s.id, s.val)
RETURNING merge_action(),
old.id IS NULL AS is_new_row;
CREATE TABLE
INSERT 0 1

merge_action | is_new_row
--------------+------------
UPDATE | f
INSERT | f -- (this should be true)
(2 rows)

MERGE 2

Thanks,
Satya