skip loading of collision geometries into rviz

[mohrad-oishii]

• Skip loading collision geometries into RViz. Collision geometries are only needed for collision checking in the planning backend, not for visualization in RViz.
• Create an empty scene node to maintain the expected structure.

[rjoomen]

Visualizing collision geometries is important for debugging, you'll be able to see what scene the motion planning works with. Deferring loading until visualization is switched on could be an option, if the current implementation really has issues regarding performance.