[1.2.X] Fixed #13882 -- Removed an unnecessary nested where clause introduced...

[1.2.X] Fixed #13882 -- Removed an unnecessary nested where clause introduced on __isnull queries. Thanks to Alex Gaynor for the report and patch.

Backport of r13456 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@13457 bcc190cf-cafb-0310-a4f2-bffc1f526a37