Write name of external generator to GenFSR also in cases where there are no interactions generated.

In some ReDecay productions with low number of events in job one can get to situation where one of the spillover slots does not have any interactions. If that happens, GenFSR is not filled which leads to problem in Brunel when merging GenFSR as during check whether all FSRs have same hardGenerator. Change here fills hardGenerator name from external generators independent whether any interaction is generated. I checked that merging with this fix succeed.

It should fix one of the issues in LHCBGAUSS-2310.

Same change might be needed in Gaussino/Gauss-on-Gaussino.