Fixed #8462 -- Made `length` and `length_is` template filters fail silently...

Fixed #8462 -- Made `length` and `length_is` template filters fail silently when given a value that has undefined length and added tests for both filters.  Based on patch from marcelor, rob, and SmileyChris.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@10193 bcc190cf-cafb-0310-a4f2-bffc1f526a37