Fixed #14876 -- Ensure that join promotion works correctly when there are...

Fixed #14876 -- Ensure that join promotion works correctly when there are nullable related fields. Thanks to simonpercivall for the report, oinopion and Aleksandra Sendecka for the original patch, and to Malcolm for helping me wrestle the edge cases to the ground.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@16648 bcc190cf-cafb-0310-a4f2-bffc1f526a37