[ATR-28358] Add hard-coded multiplicity data for HLT_id_cosmicid_L1MU11_EMPTY chain from Run 2

In ATR-28358 we are trying to test the conversion of R2 trigger navigation data to R3, including running DAOD level slimming.

The Run 3 procedure requires access to the chain's multiplicity signature, but the HLT_id_cosmicid_L1MU11_EMPTY chain is missing this in the menu summary metadata written into these MC20 AOD files by the RDO to RDO_TRIG sub-step which was running back in release 21.0 in order to execute the Run 2 trigger.

There is little we can do at this stage with the menu summary data in the AOD - so here I pragmatically add an explicit check for this case which will be used as part of the conversion (AOD->DAOD_PHYS).