Minor change in ComboHypoToolBase for anomaly detection trigger

The Run 3 anomaly detection trigger uses a ComboHypoTool with zero-multiplicity objects and thus requires this change. As this base class is used in many places, this dedicated MR has been created in addition to the original MR of the anomaly detection algorithm (!78373 (merged)).

Tagging @fwinkl , @khoo , @astruebi , @tamartin , @fpastore , @shanisch , @gipezzul , @drankin , @mmcohen