Fixed #22676 -- makemigrations --dry-run should not ask for defaults

Made the fix in InteractiveMigrationQuestioner class code, rather than
MigrationAutodetector, because --dry-run shouldn't affect whether
MigrationAutodetector will detect non-nullable fields, but the
questioner should skip the question and returns a None for default
(since that won't be used anyway) if --dry-run is used.

tests/migrations/test_migrations_no_default/0001_initial.py

@@ -0,0 +1,23 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+from django.db import models, migrations
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+    ]
+
+    operations = [
+        migrations.CreateModel(
+            name='SillyModel',
+            fields=[
+                ('id', models.AutoField(verbose_name='ID', serialize=False, auto_created=True, primary_key=True)),
+                ('silly_field', models.BooleanField(default=False)),
+            ],
+            options={
+            },
+            bases=(models.Model,),
+        ),
+    ]