Improve handling of TARGETVOLUME in VolumeDebugger

Skip running VolumeDebugger if $TARGETVOLUME is set to NONE. This will save time and avoid Warning messages for changes that don't affect the geometry (e.g. README updates, etc) in the ITKLayouts CI Tests:

https://gitlab.cern.ch/Atlas-Inner-Tracking/ITKLayouts/-/merge_requests/75