schemachanger: ADD COLUMN AS (NULL) virtual not null incorrectly succeeds #87457
Description
In the new schema changer, the following ADD COLUMN incorrectly succeeds:
SET use_declarative_schema_changer = 'unsafe_always';
CREATE TABLE t (i INT PRIMARY KEY);
INSERT INTO t VALUES (1);
ALTER TABLE t ADD COLUMN j INT AS (NULL::INT) VIRTUAL NOT NULL;
The expected behavior, as the old schema changer will give us, is
ERROR: validation of NOT NULL constraint failed: validation of CHECK "j IS NOT NULL" failed on row: i=1, j=NULL
SQLSTATE: 23514
Jira issue: CRDB-19354