[ATR-23460] Update default L1 prescale in prepareTriggerMenu.cxx
This super tiny change fixes a FPE in the 1st event in RAWtoALL jobs, in this event we are currently unfortunately unable to serve prescale values through the trig config svc due to the data not being available in beforeEvent
in the 1st event.
We will now serve -1 if asked in this event.