[4.2.x] Refs #34840 -- Fixed test_validate_nullable_textfield_with_isnull_true() on databases that don's support table check constraints.

Thanks Tim Graham for the report.
Backport of 9fd3a0ffc8 from main

tests/constraints/tests.py

@@ -779,6 +779,7 @@ class UniqueConstraintTests(TestCase):
             exclude={"name"},
         )
 
+    @skipUnlessDBFeature("supports_table_check_constraints")
     def test_validate_nullable_textfield_with_isnull_true(self):
         is_null_constraint = models.UniqueConstraint(
             "price",