[1.2.X] Fixed #4976 -- Stopped humanize template tags to raise a TypeError if...

[1.2.X] Fixed #4976 -- Stopped humanize template tags to raise a TypeError if passed a value of ``None``. Thanks, Simon G. and Adam Vandenberg.

Backport from trunk (r15000).

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15004 bcc190cf-cafb-0310-a4f2-bffc1f526a37