Fixed #6398: added an optional `{% empty %}` clause to the `{% for %}`...

Fixed #6398: added an optional `{% empty %}` clause to the `{% for %}` template tag. The contents of this clause are rendered if the list iterated over turns out to be empty. Thanks, Jannis Leidel.

Astute readers will notice that the patch originally called this `default`; after consideration I decided that `empty` is a very slightly better color for this particular bikeshed.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9530 bcc190cf-cafb-0310-a4f2-bffc1f526a37