[mini] initialize (empty) secondary vector

[SeverinDiederichs]

This initializes an empty secondary vector when returning a GPU step to the CPU. This is needed such that the access to the secondary does not crash:

  const std::vector<const G4Track*>  *secondaryVector = aStep->GetSecondaryInCurrentStep();
  secondaryVector->size(); <- currently crashes as it is a nullPtr

With this PR the secondary vector is available, just empty.

This PR is needed to run Athena with the Sim_tf transform.

[phsft-bot]

Can one of the admins verify this patch?

[SeverinDiederichs]

With the latest Athena version, this does not seem to be needed for now, but I'll keep it open for a bit to confirm with the other experiment frameworks that indeed it is no problem.

With this PR, the check for secondaries in a step would not be a nullptr access anymore, preventing potential crashes.

[SeverinDiederichs]

In fact, this is needed by CMSSW and therefore should be merged.

[JuanGonzalezCaminero]

Looks good to me