Bug#35705590 Router crashes when target is clusterset and only has access to no-quorum part.

New tests for "WL#15841 Option to allow traffic when the sole reachable
partition has no quorum" revealed a legacy issue where the Router crashes if
the target Cluster of the Router is part of a ClusterSet and the Router only has
an access to the sub-group of this cluster with no GR quorum.

The problem is the access to std::optional<> member without checking it is set,
which happens in case of Router having only access to no-quorum group.

This patch fixes the issue checking if the optional value is set before
accessing it.

This scenario is tested by WL#15841 test ClusterSetAccessToPartitionWithNoQuorum.

Change-Id: I90b3400ed5e08e03699dbb6d252755cfc93ad75f

Author: Andrzej Religa

router/src/metadata_cache/src/cluster_metadata_gr.cc

@@ -1344,17 +1344,19 @@ static std::optional<size_t> target_cluster_pos(
 
 std::optional<metadata_cache::metadata_server_t>
 GRClusterSetMetadataBackend::find_rw_server() {
-  for (auto &cluster : (*cluster_topology_).clusters_data) {
-    if (!cluster.is_primary) continue;
+  if (cluster_topology_) {
+    for (auto &cluster : (*cluster_topology_).clusters_data) {
+      if (!cluster.is_primary) continue;
 
-    log_debug("Updating the status of cluster '%s' to find the writable node",
-              cluster.name.c_str());
+      log_debug("Updating the status of cluster '%s' to find the writable node",
+                cluster.name.c_str());
 
-    // we need to connect to the Primary Cluster and query its GR status to
-    // figure out the current Primary node
-    metadata_->update_cluster_status(cluster);
+      // we need to connect to the Primary Cluster and query its GR status to
+      // figure out the current Primary node
+      metadata_->update_cluster_status(cluster);
 
-    return metadata_->find_rw_server(cluster.members);
+      return metadata_->find_rw_server(cluster.members);
+    }
   }
 
   return std::nullopt;