Fixing a bug introduced in !30540 (merged). Checking whether input files exist was added there to the HistMerge step, but it didn't correctly cover the case when we rename the input file as part of the command. Now it should do this correctly.
This fixes the ZeroCounts error seen in nearly all TrigP1Test tests today:
Tested by running one of the failing tests - test_trigP1_v1PhysP1_build.py