Updated SiTrajectoryElement to deal with uninitialized cluster collection iterator
This MR brings a small modification to the SiTrajectoryElement set method, to deal with dummy cluster collection iterator as used in the new ITk-specific pTseed method https://gitlab.cern.ch/atlas/athena/-/blob/master/InnerDetector/InDetRecEvent/SiSPSeededTrackFinderData/src/SiTrajectory_xk.cxx#L550
The change has already been checked to yield identical results with the q431 test.