post-migrate handlers fail when DATABASES['default'] = {}

[Thomas Recouvreux]

On Django 1.7.4 the management command migrate --database=xxx fails when DATABASES['default'] = {} with django.core.exceptions.ImproperlyConfigured: settings.DATABASES is improperly configured. Please supply the ENGINE value. Check settings documentation for more details..
On Django 1.7.3 it works fine.

To reproduct the problem:

1. Create a new project
2. Edit the settings.py this way:

DATABASES = {
    'default': {},
    'mysite': {
        'ENGINE': 'django.db.backends.sqlite3',
        'NAME': os.path.join(BASE_DIR, 'db_mysite.sqlite3'),
    }
}

3. Run python manage.py migrate --database=mysite

Problem:

It seems Django tries to access the default connection. I found django.db.migrations.loader.is_latest_migration_applied uses the default connection.

I am attaching an output log of the command.
Here is a sample project to reproduce the behaviour: ​https://github.com/trecouvr/test_dj_migrate_174.