[HltEfficiencyChecker]: Empy list in objects_to_persist causing crash

I was trying to test some b2cc hlt2 lines effs and found this bug. After some investigation, I discovered that the line in the link below was trying to access index 0 of an empty list. Maybe some protection is needed there.

https://gitlab.cern.ch/lhcb/DaVinci/-/blob/master/HltEfficiencyChecker/python/HltEfficiencyChecker/config.py?ref_type=heads#L201