Detect NULL default value as no default value.

This can happen when using `db_default=None` with `null=False` on a Django model field.

tests/test_project/app_add_not_null_column_with_null_db_default/migrations/0001_initial.py

@@ -0,0 +1,30 @@
+# Generated by Django 5.0.1 on 2024-02-04 20:07
+
+from __future__ import annotations
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    initial = True
+
+    dependencies = []
+
+    operations = [
+        migrations.CreateModel(
+            name="A",
+            fields=[
+                (
+                    "id",
+                    models.AutoField(
+                        auto_created=True,
+                        primary_key=True,
+                        serialize=False,
+                        verbose_name="ID",
+                    ),
+                ),
+                ("field", models.IntegerField()),
+            ],
+        ),
+    ]

tests/test_project/app_add_not_null_column_with_null_db_default/migrations/0002_a_not_null_field_with_null_db_default.py

@@ -0,0 +1,21 @@
+# Generated by Django 5.0.1 on 2024-02-04 20:07
+
+from __future__ import annotations
+
+import django.db.models.functions.math
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ("app_add_not_null_column_with_null_db_default", "0001_initial"),
+    ]
+
+    operations = [
+        migrations.AddField(
+            model_name="a",
+            name="not_null_field_db_default_null",
+            field=models.IntegerField(db_default=None, default=None, null=False),
+        ),
+    ]

tests/test_project/app_add_not_null_column_with_null_db_default/models.py

@@ -0,0 +1,9 @@
+from __future__ import annotations
+
+from django.db import models
+from django.db.models.functions import Pi
+
+
+class A(models.Model):
+    field = models.IntegerField()
+    not_null_field_db_default_null = models.IntegerField(null=False, db_default=None)