CaloThinCellsByClusterAlg and isValid check for serial run. Trying to fix e/gamma ART

In egamma ART since 28/01/2020 we observe :

ESDtoAOD 12:59:39 CaloThinCellsByClusterAlg_InDetTrackParticlesAs...  FATAL  Standard std::exception is caught
ESDtoAOD 12:59:39 CaloThinCellsByClusterAlg_InDetTrackParticlesAs...  ERROR SG::ExcNullReadHandle: Dereference of read handle failed: StoreGateSvc_Impl+InDetTrackParticlesAssociatedClusters[1219821989]
ESDtoAOD 12:59:39 AthAlgSeq                                           FATAL  Standard std::exception is caught
ESDtoAOD 12:59:39 AthAlgSeq                                           ERROR SG::ExcNullReadHandle: Dereference of read handle failed: StoreGateSvc_Impl+InDetTrackParticlesAssociatedClusters[1219821989]
ESDtoAOD 12:59:39 AthAllAlgSeq                                        FATAL  Standard std::exception is caught
ESDtoAOD 12:59:39 AthAllAlgSeq                                        ERROR SG::ExcNullReadHandle: Dereference of read handle failed: StoreGateSvc_Impl+InDetTrackParticlesAssociatedClusters[1219821989]
ESDtoAOD 12:59:39 AthAlgEvtSeq                                        FATAL  Standard std::exception is caught
ESDtoAOD 12:59:39 AthAlgEvtSeq                                        ERROR SG::ExcNullReadHandle: Dereference of read handle failed: StoreGateSvc_Impl+InDetTrackParticlesAssociatedClusters[1219821989]
ESDtoAOD 12:59:39 AthMasterSeq                                        FATAL  Standard std::exception is caught
ESDtoAOD 12:59:39 AthMasterSeq                                        ERROR SG::ExcNullReadHandle: Dereference of read handle fail

Ths prb has more to do with !29831 (merged)

I do not readily see where we change det-flags (we do though doMuons=False).

Anyhow , this adds a check for "serial" running and gives a warning when the ReadHandle is not valid.

Mentioning @turra and @dboerner

Calorimeter/CaloRec/src/CaloThinCellsByClusterAlg.cxx

@@ -40,6 +40,10 @@ StatusCode CaloThinCellsByClusterAlg::execute (const EventContext& ctx) const
   cells.thinAll();
 
   SG::ReadHandle<xAOD::CaloClusterContainer> clusters (m_clusters, ctx);
+  if(!clusters.isValid()){
+     ATH_MSG_WARNING( "Collection  " << m_clusters.key()<<" is not valid");
+     return StatusCode::SUCCESS;
+  }
   for (const xAOD::CaloCluster* clust : *clusters) {
     const CaloClusterCellLink* cellLinks = clust->getCellLinks();
     if (!cellLinks) {