Using __isnull=True on a KeyTransform should not match JSON null on SQLite and Oracle

[sage]

The KeyTransformIsNull lookup borrows the logic from HasKey for isnull=False, which is correct. If isnull=True, the query should only match objects that do not have the key. The query is correct for MariaDB, MySQL, and PostgreSQL. However, on SQLite and Oracle, the query also matches objects that have the key with the value null, which is incorrect.

To confirm, edit tests.model_fields.test_jsonfield.TestQuerying.test_isnull_key. For the first assertion, change

        self.assertSequenceEqual(
            NullableJSONModel.objects.filter(value__a__isnull=True),
            self.objs[:3] + self.objs[5:],
        )

to

        self.assertSequenceEqual(
            NullableJSONModel.objects.filter(value__j__isnull=True),
            self.objs[:4] + self.objs[5:],
        )

The test previously only checks with value__a which could not catch this behavior because the value is not JSON null.

[sage]

​PR.

[Mariusz Felisiak <felisiak.mariusz@…>]

In 8d7085e0:

Fixed #32252 -- Fixed isnull=True on key transforms on SQLite and Oracle.

isnull=True on key transforms should not match keys with NULL values.

[Mariusz Felisiak <felisiak.mariusz@…>]

In a891e1b:

[3.1.x] Fixed #32252 -- Fixed isnull=True on key transforms on SQLite and Oracle.

isnull=True on key transforms should not match keys with NULL values.

Backport of 8d7085e0fd004af5431389f3d903aba6220d7957 from master