Fixed #10089 -- Corrected handling of aggregates when the query set contains...

Fixed #10089 -- Corrected handling of aggregates when the query set contains no items (and the cursor returns None). Thanks to Kyle Fox for the report, and david for the initial patch.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@9786 bcc190cf-cafb-0310-a4f2-bffc1f526a37