Fixed #10594 -- `GeoQuerySet` measurment methods no longer crash on geometry...

Fixed #10594 -- `GeoQuerySet` measurment methods no longer crash on geometry fields with NULL values.  Thanks, whiteinge for the bug report and yourcelf for the initial patch.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@12885 bcc190cf-cafb-0310-a4f2-bffc1f526a37