Protection against negative jet masses for combined mass

In rare cases when the jet has only one constituent (i.e. calorimeter jet mass of zero by default), the jet mass can be negative (very small value) after the calibration has been applied due to storing it as a TLorentzVector in between, etc. When the combined jet mass is calculated, the tool will return a jet mass resolution of zero to protect against negative values when reading the histograms. In the combination, a resolution of zero means however that all weight is given to that specific mass definition instead of using the well-defined other mass definition (for example track-assisted jet mass). Therefore, a protection was added in case the resolution function returns zero, to set the corresponding mass definition to zero.