Fix for ATLINFR-5383 : only check for files for DiffPool

Closes ATLINFR-5383

I'm really not sure how we didn't run into this before, but this adds a protection to not check for file paths if they shouldn't exist (as uncovered on !71511 (merged))

cc @abarton

mentioned in merge request !71511 (merged)

This merge request affects 1 package:

• Tools/PROCTools

This merge request affects 1 file:

• Tools/PROCTools/python/update_ci_reference_files.py

added Tools main review-pending-level-1 labels

enabled an automatic merge when the pipeline for b4381b80 succeeds

CI Result SUCCESS (hash b4381b80)

	Athena	AthSimulation
externals
cmake
make
tests

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AthSimulation: number of compilation errors 0, warnings 0
For experts only: Jenkins output [CI-MERGE-REQUEST-EL9 10103] (remote access info)

merged

mentioned in commit 0df17962