InputConverter.cxx: fix for HepMC2-based running (ATLASSIM-7418)

John Derek Chapmanrequested to merge
jchapman/athena:InputConverterHepMC2_main into main

This fix only affects compilation against HepMC2, so the CI (which uses HepMC3) will see no effect.

It should fix the warning highlighted in ATLASSIM-7418 though.

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