Protection for correctly computing the bounding box when adding assemblies without content to another assembly.

(cherry picked from commit eeb676568becee8d5eb1de3e2a7c62bfa301bab8)
Fixed broken comment in TGeoVolume.cxx preventing displaying correctly the reference guide

(cherry picked from commit 329eedd11a2c688f01857dbcf39f6b6094d21f7c)