Ensure that we are correctly accounting for events in with zero generation probability

[nickkamp1]

Make sure that events with zero generation probability (e.g., no available interactions) will lead to the proper enhancement in the generation probability of the other events (leading to a decrease in the physical probability)

[nickkamp1]

Note that the weights are already properly calculated if we restrict to generations that try to generate N events and return only the physical subset of those events, and then calculate weights based on the N requested events.