See also tests that manipulates with NULL fields/domains and check results:
Adding a non-null restricted column to a populated table renders the table inconsistent;
(Regression: NOT NULL constraint, declared in domain, does not work);
(Inconsistencies with ALTER DOMAIN and ALTER TABLE with DROP NOT NULL and PRIMARY KEYs);
(Command "Alter table <T> alter TYPE <C> <DOMAIN_WITH_NOT_NULL" does not verifies data in column <C> and makes incorrect assignments in <C> to ZERO / JULIAN_DATE / ASCII(0) for types INT, TIMESTAMP and VARCHAR).