rigidbody: Don't embed collision margin if object has no volume

While it's not a good idea to create convex hull shapes from objects
with no volume, this makes them behave a little nicer.

Fixes [#34410] Planes with Rigid Body always keep distance to colliding objects

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@54855 954f8c5b-7b00-dc11-b283-0030488c597c