BUG #18634: Wrong varnullingrels with merge ... when not matched by source

The following bug has been logged on the website:

Bug reference:      18634
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 17.0
Operating system:   Ubuntu 22.04
Description:

The following script:
CREATE TABLE t (a int);
INSERT INTO t VALUES(1), (2);
CREATE VIEW v AS SELECT a FROM t WHERE EXISTS (SELECT 1 FROM t);

MERGE INTO v USING (SELECT * FROM generate_series(1,1)) AS s(a)
  ON s.a = v.a WHEN NOT MATCHED BY SOURCE THEN DELETE;

produces:
ERROR:  XX000: wrong varnullingrels (b) (expected (b 4)) for Var 5/1
LOCATION:  search_indexed_tlist_for_var, setrefs.c:2847

At the same time,
MERGE INTO v USING (SELECT * FROM generate_series(1,1)) AS s(a)
  ON s.a = v.a WHEN MATCHED THEN DELETE;
planned and executed with no error.