Fixing issues with MET validation after AOD update
Currently the validation framework is crashing when running doMET over the new AODs that do not contain the relevant MET containers anymore. It was specifically requested by @jferrand that the code would not crash but just not fill the histograms. This MR takes care of this. It furthermore fixes crashes that appear when running over the new style derivations because we do not build MET_Reference anymore.
This MR is somewhat related to ATR-23806