Add forward validator to forward then matching sequence

While debugging an optimisation of the prefix sum in CreateReducedSciFiHitContainer, I found that the used_hits array is always empty. I added this validator and realized that the forward is not producing any tracks in the hlt1_pp_forward_then_matching sequence on Beam6800GeV-expected-2024-MagDown-nu7.6 sample.

Found the issue: I was using the wrong geometry, /scratch/allen_data/mdf_input/Beam6800GeV-expected-2024-MagDown-nu7.6/geometry_dddb-20231017_sim-20231017-vc-md100 instead of /scratch/allen_geometries/geometry_dddb-20231017_sim-20231017-vc-md100_new_SciFi_geometry/ for my local tests. However I think having this extra validator could help catch issues that are otherwise invisible.