BUG #19630: FOR PORTION OF crashes with XX000 when target range is NULL

The following bug has been logged on the website:

Bug reference: 19630
Logged by: Zheng Hacker
Email address: hackerzheng666(at)gmail(dot)com
PostgreSQL version: 19beta3
Operating system: Linux x86_64
Description:

PostgreSQL version: 20devel (commit bdbf662, 2026-08-19)
OS: Linux x86_64

When a DELETE/UPDATE ... FOR PORTION OF receives a NULL range
target, it hits elog(ERROR) without errcode() in nodeModifyTable.c,
producing SQLSTATE XX000 instead of a proper error.

Reproducer:

CREATE TABLE t (
id int4range NOT NULL,
valid_at daterange NOT NULL,
name text NOT NULL,
CONSTRAINT t_pk PRIMARY KEY (id, valid_at WITHOUT OVERLAPS)
);
INSERT INTO t (id, valid_at, name)
VALUES ('[1,2)', '[2018-01-02,2020-01-01)', 'one');

DELETE FROM t FOR PORTION OF valid_at (NULL) WHERE id = '[1,2)';
-- ERROR: XX000: FOR PORTION OF target was null
-- LOCATION: ExecInitModifyTable, nodeModifyTable.c:5647

Expected: a proper SQLSTATE (e.g. 22004 null_value_not_allowed).

Found by automated SQL fuzzing.
Credit: Zheng Wang, Yanjie Zhao, Yiyang Liu