btCollisionObjects w/o a user ptr can collide
btCollisionObjects w/o a user ptr can collide

We set the user pointer on all btCollisionObjects that are part of the
collision model, but when evaluating point-cloud to model collisions, we create
new btCollisionObjects which do not have a set user pointer because they don't
belong to a DrakeCollision::Element. These should still be able to collide with
the model.

scottviteri/drake,scottviteri/drake,crogers3/drake,thientu/drake,psiorx/drake,thientu/drake,crogers3/drake,psiorx/drake,thientu/drake,hanssusilo/drake,hanssusilo/drake,edowson/drake,edowson/drake,hanssusilo/drake,edowson/drake,thientu/drake,psiorx/drake,billhoffman/drake,scottviteri/drake,crogers3/drake,billhoffman/drake,hanssusilo/drake,psiorx/drake,thientu/drake,edowson/drake,hanssusilo/drake,billhoffman/drake,edowson/drake,billhoffman/drake,scottviteri/drake,crogers3/drake,sheim/drake,billhoffman/drake,sheim/drake,scottviteri/drake,crogers3/drake,psiorx/drake,hanssusilo/drake,sheim/drake,scottviteri/drake,psiorx/drake,sheim/drake,sheim/drake,edowson/drake,edowson/drake,psiorx/drake,billhoffman/drake,crogers3/drake,crogers3/drake,crogers3/drake,thientu/drake,billhoffman/drake,billhoffman/drake,edowson/drake,hanssusilo/drake,psiorx/drake,scottviteri/drake,hanssusilo/drake,sheim/drake,sheim/drake,thientu/drake,sheim/drake