Added empty mail_admins filter to prevent DeprecationWarning

The manage.py functions warn about missing mail_admins filter:

# python manage.py validate
/usr/local/lib/python2.6/dist-packages/django/conf/__init__.py:219: DeprecationWarning: You have no filters defined on the 'mail_admins' logging handler: adding implicit debug-false-only filter. See http://docs.djangoproject.com/en/dev/releases/1.4/#request-exceptions-are-now-always-logged
  DeprecationWarning)
0 errors found

An empty filter allows these to run cleanly.