PyJobTransforms: Fix RNTuple validation in main--dev3LCG

Alaettin Serhan Meterequested to merge
amete/athena:main-rntuple-validation-update-20250113 into main

Yet another API change, similar to !76658 (merged). After root/pull/17369 reading an RNTuple in python is ever so slightly different.

cc: @gemmeren

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