ATLPHYSVAL-771: fix the InDetPhysValMonitoring efficiency plot filling
This MR fixes a bug introduced in !44704 (merged), which led to an error being printed for every non-reconstructed truth track in the efficiency calculation of InDetPhysValMonitoring.
To keep cppchecker happy, we move from a reference to a pointer when passing the (possibly non-existent) reconstructed track.
Should fix the errors in https://its.cern.ch/jira/browse/ATLPHYSVAL-771 - adding "urgent" label to unblock this validation step.
Edited by Maximilian Emanuel Goblirsch-Kolb