Fixed #26135 -- Adjusted the migration questioner's handling of disabled apps.

This was causing an issue when calling the `migrate` command in a test case with
the `available_apps` attribute pointing to an application with migrations
disabled using the `MIGRATION_MODULES` setting.

Thanks to Tim Graham for the review.

Refs #24919

tests/migrations/test_questioner.py

@@ -0,0 +1,15 @@
+from __future__ import unicode_literals
+
+from django.db.migrations.questioner import MigrationQuestioner
+from django.test import SimpleTestCase
+from django.test.utils import override_settings
+
+
+class QuestionerTests(SimpleTestCase):
+    @override_settings(
+        INSTALLED_APPS=['migrations'],
+        MIGRATION_MODULES={'migrations': None},
+    )
+    def test_ask_initial_with_disabled_migrations(self):
+        questioner = MigrationQuestioner()
+        self.assertIs(False, questioner.ask_initial('migrations'))