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Fix LLP1 RCSubstructureAlg for very rare possibility where a jet has no constituents after grooming

Guillaume Lucas Albouy requested to merge galbouy/athena:DAOD_LLP1_dev into main

A rare bug was observed during data23 LLP1 derivation processing, see ATLASDPD-1972) and one derivation log with the error. Log snippet with error:

22:59:27 fastjet::Error:  EnergyCorrelator called on jet with no constituents.
22:59:27 2023-10-05 22:57:33,069 RCSubstructureC...  FATAL UNKNOWN Exception is caught
22:59:27 2023-10-05 22:57:33,111 AthAlgSeq           FATAL UNKNOWN Exception is caught
22:59:27 2023-10-05 22:57:33,111 AthAllAlgSeq        FATAL UNKNOWN Exception is caught
22:59:27 2023-10-05 22:57:33,112 AthAlgEvtSeq        FATAL UNKNOWN Exception is caught
22:59:27 2023-10-05 22:57:33,112 AthMasterSeq        FATAL UNKNOWN Exception is caught

It occurred in RCSubstructureAlg and seems to happen in really rare cases where a jet reconstructed from topo clusters has no constituents left after applying some grooming. MR implements a fix by checking number of jet constituents after grooming procedure.

Tagging @emmat, @delsart.

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