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

Thanks Brian Ibbotson for the report.

Regression in 7414704e88.

Backport of 1570ef02f3 from main.

django/db/models/base.py

@@ -30,7 +30,7 @@ from django.db.models import NOT_PROVIDED, ExpressionWrapper, IntegerField, Max,
 from django.db.models.constants import LOOKUP_SEP
 from django.db.models.constraints import CheckConstraint, UniqueConstraint
 from django.db.models.deletion import CASCADE, Collector
-from django.db.models.expressions import RawSQL
+from django.db.models.expressions import DatabaseDefault, RawSQL
 from django.db.models.fields.related import (
     ForeignObjectRel,
     OneToOneField,
@@ -1568,6 +1568,9 @@ class Model(AltersData, metaclass=ModelBase):
             raw_value = getattr(self, f.attname)
             if f.blank and raw_value in f.empty_values:
                 continue
+            # Skip validation for empty fields when db_default is used.
+            if isinstance(raw_value, DatabaseDefault):
+                continue
             try:
                 setattr(self, f.attname, f.clean(raw_value, self))
             except ValidationError as e: