Update the TruthSvc to work with ShadowEvents

Builds upon !63457 (merged).

This MR updates the TruthSvc to deal with the case that the GenParticle and GenVertex objects associated with decays of quasi-stable particles predefined by the generator may not be present in the output GenEvent. This is ability is switched off by default, so no change in output is expected.

Part of the changes from !63059 (merged).

Tagging @mbandier, @tlari, @averbyts, @pclark

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