add testcase for DROP NOT NULL

tests/test_project/app_alter_column_drop_not_null/admin.py

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+from django.contrib import admin
+
+# Register your models here.

tests/test_project/app_alter_column_drop_not_null/apps.py

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+from django.apps import AppConfig
+
+
+class AppAddNotNullColumnConfig(AppConfig):
+    name = "app_alter_column_drop_not_null"

tests/test_project/app_alter_column_drop_not_null/migrations/0001_create_table.py

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+# Generated by Django 2.1.4 on 2019-03-19 20:59
+
+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",
+                    ),
+                ),
+                ("not_null_field", models.IntegerField(null=False)),
+            ],
+        )
+    ]

tests/test_project/app_alter_column_drop_not_null/migrations/0002_drop_not_null.py

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+# Generated by Django 2.1.4 on 2019-03-19 21:05
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [("app_alter_column_drop_not_null", "0001_create_table")]
+
+    operations = [
+        migrations.AlterField(
+            model_name='a',
+            name='not_null_field',
+            field=models.IntegerField(null=True),
+        ),
+    ]

tests/test_project/app_alter_column_drop_not_null/models.py

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+from django.db import models
+
+
+class A(models.Model):
+    not_null_field = models.IntegerField(null=True)

tests/test_project/app_alter_column_drop_not_null/tests.py

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+from django.test import TestCase
+
+# Create your tests here.

tests/test_project/app_alter_column_drop_not_null/views.py

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+from django.shortcuts import render
+
+# Create your views here.