[4.2.x] Fixed #34754 -- Fixed JSONField check constraints validation on NULL values.

The __isnull lookup of JSONField must special case
Value(None, JSONField()) left-hand-side in order to be coherent with
its convoluted null handling.

Since psycopg>=3 offers no way to pass a NULL::jsonb the issue is
resolved by optimizing IsNull(Value(None), True | False) to
True | False.

Regression in 5c23d9f0c32f166c81ecb6f3f01d5077a6084318.

Thanks Alexandre Collet for the report.

Backport of 3434dbd39d373df7193ad006b970c09c1a909ea3 from main

2 files changed, 27 insertions, 0 deletions

diff --git a/tests/constraints/models.py b/tests/constraints/models.py
index 3b349d204e..939efe0b83 100644
--- a/tests/constraints/models.py
+++ b/tests/constraints/models.py
@@ -115,3 +115,10 @@ class AbstractModel(models.Model):
 
 class ChildModel(AbstractModel):
     pass
+
+
+class JSONFieldModel(models.Model):
+    data = models.JSONField(null=True)
+
+    class Meta:
+        required_db_features = {"supports_json_field"}
diff --git a/tests/constraints/tests.py b/tests/constraints/tests.py
index e52d15233c..5f59b3a47e 100644
--- a/tests/constraints/tests.py
+++ b/tests/constraints/tests.py
@@ -11,6 +11,7 @@ from django.test import SimpleTestCase, TestCase, skipIfDBFeature, skipUnlessDBF
 from .models import (
     ChildModel,
     ChildUniqueConstraintProduct,
+    JSONFieldModel,
     Product,
     UniqueConstraintConditionProduct,
     UniqueConstraintDeferrable,
@@ -251,6 +252,25 @@ class CheckConstraintTests(TestCase):
         )
         constraint.validate(Product, Product())
 
+    @skipUnlessDBFeature("supports_json_field")
+    def test_validate_nullable_jsonfield(self):
+        is_null_constraint = models.CheckConstraint(
+            check=models.Q(data__isnull=True),
+            name="nullable_data",
+        )
+        is_not_null_constraint = models.CheckConstraint(
+            check=models.Q(data__isnull=False),
+            name="nullable_data",
+        )
+        is_null_constraint.validate(JSONFieldModel, JSONFieldModel(data=None))
+        msg = f"Constraint “{is_null_constraint.name}” is violated."
+        with self.assertRaisesMessage(ValidationError, msg):
+            is_null_constraint.validate(JSONFieldModel, JSONFieldModel(data={}))
+        msg = f"Constraint “{is_not_null_constraint.name}” is violated."
+        with self.assertRaisesMessage(ValidationError, msg):
+            is_not_null_constraint.validate(JSONFieldModel, JSONFieldModel(data=None))
+        is_not_null_constraint.validate(JSONFieldModel, JSONFieldModel(data={}))
+
 
 class UniqueConstraintTests(TestCase):
     @classmethod