[1.2.X] Fixed #15336 -- Silenced a warning about the DATABASES transition when...

[1.2.X] Fixed #15336 -- Silenced a warning about the DATABASES transition when a database backend isn't specified. Thanks to Ivan Sagalaev for the report and patch.

Backport of r15571 from trunk.

git-svn-id: http://code.djangoproject.com/svn/django/branches/releases/1.2.X@15575 bcc190cf-cafb-0310-a4f2-bffc1f526a37