[1.3.X] Fixed #14876 -- Ensure that join promotion works correctly when there...

[1.3.X] 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.

Backport of r16648 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.3.X@16671 bcc190cf-cafb-0310-a4f2-bffc1f526a37