Fixes on TrigCaloCellMakerMT and egammaSuperClusterBuilder for TrigEgamma

This MR is to fix 2 issues affecting TrigEgamma photon HLT chain at precision reconstruction as discussed in ATR-18929:

1. In order for the cells used to build the clusters at fast reconstruction the method finalizeCluster needs to be executed in TrigCaloClusterMakerMT
2. For some reason, even after that is done, the getCellLinks() will give a nullptr, making the egammaSuperClusterBuilder to just crash when accessing the cells. A simple replacement of that function by using directly the ElementLink Accesor is proposed by experts and implemted here.