Improve memory management of fastjet::ClusterSequence in JetClusterer
@smh reported a crash that occurs when there are no jets found, and we attempt to call fastjet::ClusterSequence::delete_self_when_unused().
This should address that by exiting safely, returning an empty jet container & store, if we find no jets. Moved the ClusterSequence into a unique_ptr for robustness, handing over ownership if we successfully call the delete_self_when_unused method.