Fix MM digi identifier warnings (ATLASSIM-5822)
Fixed an issue in Micromega digitization where digits were sometimes assigned an incorrect identifier. The identifier in question was always a MDT identifier and caused some digits to be discarded.
The underlying issue was caused by an identifier being initialized and not always assigned to a value before being used. We have made sure that the identifier is now defined with a value.
The issue also came from HITS with exactly 0 kinetic energy. The MM experts decided it was best to also simply ignore these HITS as they may cause issues in the logic of the ionization simulations. These HITS so far only seem to appear in pileup jobs and the rate of such particles is very small to be negligible to the end result (about a hundred pileup HITS per 1000 events).