Prevent setting a column as identity if its not-null constraint is invalid

We don't allow null values to appear in identity-generated columns in
other ways, so we shouldn't let unvalidated not-null constraints do it
either.  Oversight in commit a379061a22.

Author: jian he <jian.universality@gmail.com>
Backpatch-through: 18
Discussion: https://postgr.es/m/CACJufxGQM_+vZoYJMaRoZfNyV=L2jxosjv_0TLAScbuLJXWRfQ@mail.gmail.com

src/test/regress/expected/constraints.out

@@ -1404,6 +1404,10 @@ ALTER TABLE notnull_tbl1 ADD PRIMARY KEY (a);
 ERROR:  cannot create primary key on column "a"
 DETAIL:  The constraint "nn" on column "a" of table "notnull_tbl1", marked NOT VALID, is incompatible with a primary key.
 HINT:  You might need to validate it using ALTER TABLE ... VALIDATE CONSTRAINT.
+-- cannot set column as generated-as-identity if it has an invalid not-null
+ALTER TABLE notnull_tbl1 ALTER COLUMN a ADD GENERATED ALWAYS AS IDENTITY;
+ERROR:  incompatible NOT VALID constraint "nn" on relation "notnull_tbl1"
+HINT:  You might need to validate it using ALTER TABLE ... VALIDATE CONSTRAINT.
 -- ALTER column SET NOT NULL validates an invalid constraint (but this fails
 -- because of rows with null values)
 ALTER TABLE notnull_tbl1 ALTER a SET NOT NULL;

src/test/regress/sql/constraints.sql

@@ -832,6 +832,9 @@ ALTER TABLE notnull_tbl1 ADD CONSTRAINT nn NOT NULL a;
 -- cannot add primary key on a column with an invalid not-null
 ALTER TABLE notnull_tbl1 ADD PRIMARY KEY (a);
 
+-- cannot set column as generated-as-identity if it has an invalid not-null
+ALTER TABLE notnull_tbl1 ALTER COLUMN a ADD GENERATED ALWAYS AS IDENTITY;
+
 -- ALTER column SET NOT NULL validates an invalid constraint (but this fails
 -- because of rows with null values)
 ALTER TABLE notnull_tbl1 ALTER a SET NOT NULL;