Change to JetUncertainties treatment of negative mass jets

JetCalibTools and JetUncertainties currently have different methods of treating the combined mass of jets where both of the input masses are negative. This is an extreme case, but it should be properly handled in order to differentiate between input problems vs different conventions.

This MR changes JetUncertainties to now accept events where the two input mass definitions (calo and TA) are both negative, and the combined mass value is 0. In contrast, if the calo and TA masses are both negative while the combined mass value is non-zero, then this is a different problem that is unrelated to JetCalibTools and most likely signifies a problem in the user input file.