rigidbody: Properly handle constrained objects not having rigid bodies

This is a pretty rare case that can be triggered by switching rigid body
and constraint groups before simulation was validated.
Code checked for existing physics objects but was missing else block.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54680 954f8c5b-7b00-dc11-b283-0030488c597c