message: [TFXLA] Return nullopt if no merge node found.
diff: @@ -1067,6 +1067,10 @@ FunctionalizeCond::CreateCorrespondingMergeCluster(Cluster* switch_cluster) {
  enqueue_or_update_merge(out);
  }
  }
  + // Return if there are no merge nodes.
  + if (merges.empty()) {
  + return gtl::nullopt;
  + }
  auto it = merges.begin();
  Cluster* merge_cluster = *it;
  for (++it; it != merges.end(); ++it) {
  