Fixed #35223 -- Made Model.full_clean() ignore fields with db_default when validating empty values.

Thanks Brian Ibbotson for the report.

Regression in 7414704e88.

tests/field_defaults/tests.py

@@ -2,6 +2,7 @@ from datetime import datetime
 from decimal import Decimal
 from math import pi
 
+from django.core.exceptions import ValidationError
 from django.db import connection
 from django.db.models import Case, F, FloatField, Value, When
 from django.db.models.expressions import (
@@ -169,6 +170,23 @@ class DefaultTests(TestCase):
         years = DBDefaultsFunction.objects.values_list("year", flat=True)
         self.assertCountEqual(years, [2000, datetime.now().year])
 
+    def test_full_clean(self):
+        obj = DBArticle()
+        obj.full_clean()
+        obj.save()
+        obj.refresh_from_db()
+        self.assertEqual(obj.headline, "Default headline")
+
+        obj = DBArticle(headline="Other title")
+        obj.full_clean()
+        obj.save()
+        obj.refresh_from_db()
+        self.assertEqual(obj.headline, "Other title")
+
+        obj = DBArticle(headline="")
+        with self.assertRaises(ValidationError):
+            obj.full_clean()
+
 
 class AllowedDefaultTests(SimpleTestCase):
     def test_allowed(self):