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

django/db/migrations/questioner.py

@@ -38,6 +38,9 @@ class MigrationQuestioner(object):
         except LookupError:         # It's a fake app.
             return self.defaults.get("ask_initial", False)
         migrations_import_path = MigrationLoader.migrations_module(app_config.label)
+        if migrations_import_path is None:
+            # It's an application with migrations disabled.
+            return self.defaults.get("ask_initial", False)
         try:
             migrations_module = importlib.import_module(migrations_import_path)
         except ImportError:

docs/releases/1.9.2.txt

@@ -71,3 +71,7 @@ Bugfixes
 
 * Fixed a crash when using a reverse ``OneToOneField`` in
   ``ModelAdmin.readonly_fields`` (:ticket:`26060`).
+
+* Fixed a crash 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 (:ticket:`26135`).

tests/migrations/test_questioner.py

+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'))