[1.1.X] Fixed #10594 -- `GeoQuerySet` measurment methods no longer crash on...

[1.1.X] 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.

Backport of r12885 from trunk.


git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.1.X@12886 bcc190cf-cafb-0310-a4f2-bffc1f526a37