Fixed #16676 -- Corrected the behavior of the 'add' template filter to return an empty string instead of the given value unchanged in the case of an invalid use. Thanks to dtrebbien for the report and to Aymeric Augustin for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16851 bcc190cf-cafb-0310-a4f2-bffc1f526a37