MuonEfficiencyCorrections Replicas filled with null

This MR solves a couple of bugs in MuonEfficiencyCorrections:

-one for SF replicas -one for readout of the JetDR decoration from derivations (which was causing a crash on new derivations)

thanks to @rhawking for spotting the mistake which has been sitting there for a long long time.

Adding: @szambitio, @nkoehler, @gartoni

Cheers, Johannes