Refs #13167 -- Corrected a regression in the way non-existent variables are...

Refs #13167 -- Corrected a regression in the way non-existent variables are handled by {% if %} tags. Thanks to ohmi2 for pointing out the regression in 1.2, and Karen for the patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@12954 bcc190cf-cafb-0310-a4f2-bffc1f526a37