RootAuxDynIO: Fix getting field info in RNTupleReader
It looks like we introduced a bug in !67755 (merged). If a field doesn't exist in the input file, dereferencing the field pointer unconditionally causes a crash. I noticed this in the RDOtoRDOTrigger step of test_mc21_overlay_trigger_reconstruction_rntuple.sh while working on !67687 (merged).
I also got rid of attr_name, which we were not using by the looks of it.