schemahcl: handle null values as omission #1839

Job	Run time
cli	3m 2s
unit (1.22)	25s
integration	35s
generate-cmp	42s
lint	39s
dialect-integration / integration-mysql57	2m 49s
dialect-integration / integration-maria102	2m 52s
dialect-integration / integration-mysql8	3m 8s
dialect-integration / integration-maria107	2m 47s
dialect-integration / integration-mysql56	2m 53s
dialect-integration / integration-maria103	2m 44s
dialect-integration / integration-postgres10	3m 5s
dialect-integration / integration-postgres11	3m 9s
dialect-integration / integration-postgres12	3m 10s
dialect-integration / integration-postgres13	3m 6s
dialect-integration / integration-postgres-ext-postgis	3m 9s
dialect-integration / integration-postgres14	3m 4s
dialect-integration / integration-postgres15	3m 9s
dialect-integration / integration-sqlite	2m 33s
dialect-integration / integration-tidb5	3m 22s
dialect-integration / integration-cockroach	2m 53s
dialect-integration / integration-tidb6	3m 25s
	56m 41s

Provide feedback