Support setting table and column comments to `NULL` #1853

Job	Run time
type generation	15s
test (pg: 14.8, schema: public)	43s
lint	9s
dead code check	17s
test (pg: 14.8, schema: non_public)	54s
test (pg: 15.3, schema: public)	55s
validate examples JSON	18s
test (pg: 15.3, schema: non_public)	44s
license check	7s
test (pg: 16.0, schema: public)	1m 6s
test (pg: 16.0, schema: non_public)	1m 5s
test (pg: latest, schema: public)	55s
examples (pg: 14.8, schema: public)	50s
test (pg: latest, schema: non_public)	59s
examples (pg: 14.8, schema: non_public)	57s
examples (pg: 15.3, schema: public)	49s
examples (pg: 15.3, schema: non_public)	45s
examples (pg: 16.0, schema: public)	49s
examples (pg: 16.0, schema: non_public)	44s
examples (pg: latest, schema: public)	44s
examples (pg: latest, schema: non_public)	47s
release	0s
	14m 52s

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