Do not decode energy thresholds if they are absent in menu

This is fix for: issue discussed ATR-24808 and blocking: ATR-25334 In practice, it avoids decoding energy (MET significance, XS, in particular) when they are absent in the menu. Apparently, missing thresholds in the menu does not mean that some sort of overflow/instability appears and the data looks like containing XS thresholds. Tagging @watsona @tamartin @stelzer (a small change in L1Menu API behaviour). FI @nova @todorova