[4.1.x] Fixed #33996 -- Fixed CheckConstraint validation on NULL values.

Bug in 667105877e.

Thanks James Beith for the report.

Backport of e14d08cd89 from main

tests/constraints/tests.py

@@ -6,7 +6,7 @@ from django.db.models import F
 from django.db.models.constraints import BaseConstraint
 from django.db.models.functions import Lower
 from django.db.transaction import atomic
-from django.test import SimpleTestCase, TestCase, skipUnlessDBFeature
+from django.test import SimpleTestCase, TestCase, skipIfDBFeature, skipUnlessDBFeature
 
 from .models import (
     ChildModel,
@@ -234,6 +234,23 @@ class CheckConstraintTests(TestCase):
         constraint.validate(Product, Product(price=501, discounted_price=5))
         constraint.validate(Product, Product(price=499, discounted_price=5))
 
+    @skipUnlessDBFeature("supports_comparing_boolean_expr")
+    def test_validate_nullable_field_with_none(self):
+        # Nullable fields should be considered valid on None values.
+        constraint = models.CheckConstraint(
+            check=models.Q(price__gte=0),
+            name="positive_price",
+        )
+        constraint.validate(Product, Product())
+
+    @skipIfDBFeature("supports_comparing_boolean_expr")
+    def test_validate_nullable_field_with_isnull(self):
+        constraint = models.CheckConstraint(
+            check=models.Q(price__gte=0) | models.Q(price__isnull=True),
+            name="positive_price",
+        )
+        constraint.validate(Product, Product())
+
 
 class UniqueConstraintTests(TestCase):
     @classmethod