Crash with finite S0 and --generatePrimariesOnly

In the case of finite S0 in the beam distribution and using the --generatePrimariesOnly there is no beam line to query for the transform. This is meant to exit safely, however it crashes.

Aside from this we can generate safely without the S0 transform without exiting - we should just print a warning in this case.

This would be used to validate the distribution started at some point in the lattice so it's ok to not use the S0 in this case.