G4 produces multiple particles with the same TrackID when called from generators that may not produce a particle.
When running multiple generators (including a radio logical generator which infrequently contributes to the event), the G4 Stage calculates the TrackIDOffset incorrectly when going from the generator without a particle to one with a particle. This code issue for this bug has already been identified in collaboration with Tom Junk.
Small change to ParticleListAction to assure that the fTrackIDOffset is only changed when in fact a track exists from a given generator. This addressed LArSoft issue #16641.