Ensure that GenEvent barcodes attribute is filled after GenEvent copy constructor is called.

This fixes MC Overlay in the master--HepMC3 nightlies. It is urgently needed for the HepMC3 validation for MC23. Fix for ATLASSIM-6152.

We are working on a follow up MR with a more (CPU) efficient implementation.

Tagging @averbyts, @tsulaia, @tadej.