[1.2.X] Fixed #12687 -- fixed an issue with aggregates and counts in...

[1.2.X] Fixed #12687 -- fixed an issue with aggregates and counts in conjunction with annotations where the QuerySet was provably empty.  Backport of [14586].

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@14587 bcc190cf-cafb-0310-a4f2-bffc1f526a37