Update TPileupReweighting.cxx to work with low-cross-section signal samples

Update TPileupReweighting.cxx so that it does not enforce a minimum normalization, but rather checks that a histogram does not have an integral of exactly zero.

The current setup gives pathological values for samples with very low signal cross-section. The proposed solution would be more general for all cross-section values while still avoiding seg faults due to dividing by integral of histograms with 0-weights.