Fixed #32425 -- Fixed adding nullable field with default on MySQL.

Thanks Simon Charette for the review.

tests/schema/tests.py

@@ -3095,6 +3095,33 @@ class SchemaTests(TransactionTestCase):
             if connection.features.can_introspect_default:
                 self.assertIsNone(field.default)
 
+    def test_add_field_default_nullable(self):
+        with connection.schema_editor() as editor:
+            editor.create_model(Author)
+        # Add new nullable CharField with a default.
+        new_field = CharField(max_length=15, blank=True, null=True, default='surname')
+        new_field.set_attributes_from_name('surname')
+        with connection.schema_editor() as editor:
+            editor.add_field(Author, new_field)
+        Author.objects.create(name='Anonymous1')
+        with connection.cursor() as cursor:
+            cursor.execute('SELECT surname FROM schema_author;')
+            item = cursor.fetchall()[0]
+            self.assertIsNone(item[0])
+            field = next(
+                f
+                for f in connection.introspection.get_table_description(
+                    cursor,
+                    'schema_author',
+                )
+                if f.name == 'surname'
+            )
+            # Field is still nullable.
+            self.assertTrue(field.null_ok)
+            # The database default is no longer set.
+            if connection.features.can_introspect_default:
+                self.assertIn(field.default, ['NULL', None])
+
     def test_alter_field_default_dropped(self):
         # Create the table
         with connection.schema_editor() as editor: