Set the prescale value to -1 when the chain is disabled (ATR-25967)

Changes in RatesTrigger.cxx and RatesAnalysisAlg.cxx, tested with:

asetup Athena,23.0,r2023-04-03T2101
test_trigP1_v1PhysP1_T0MonTrf_build.py

The changes in RatesTrigger also fix the FPE DIVBYZERO warnings for ATR-27151.

Tagging @aporeba