set cross section scale in writer even if no events are written

This ensures that the final cross section scale in the writer is
correct, even if the last input events do not produce any output events.
This ensures the correct final cross section in LHEF-based output
formats, but _not_ in HepMC-type ones. It also removes a few superfluous
updates if more than one resummation event is generated for a single
input event.