WIP: skip events with empty MC files in Checker
- when running the PVChecker with an empty MC truth file, it would crash
- same for TrackChecker
- now these events are skipped
I tested with the Bsphiphi sample that I still get the same PV efficiencies after the change and verified that it would not crash anymore when running over the Zmumu sample
cc @gligorov
Edited by Florian Reiss