Incorrect Matching of Null Values in JSONField Lookups on SQLite and Oracle

[archie-ai-code-explain-pr-review]

• The KeyTransformIsNull lookup on SQLite and Oracle incorrectly matches objects that have the key with the value null, when the query should only match objects that do not have the key.
• The expected behavior is that the query NullableJSONModel.objects.filter(value__j__isnull=True) should return the objects with indices [0, 1, 2, 3, 5, 6, 7], but on SQLite and Oracle, it also includes the object with index 4, which has the key j with a null value.
• This issue is present in the Django test tests.model_fields.test_jsonfield.TestQuerying.test_isnull_key, and the test can be updated to catch this behavior by changing the first assertion to check the value__j__isnull=True query instead of value__a__isnull=True.