Check if INSTANCE_GROUPS is empty in detect_node_failures.

cluster/log-dump/log-dump.sh

@@ -435,6 +435,9 @@ function detect_node_failures() {
   fi
 
   detect-node-names
+  if [ -z "$INSTANCE_GROUPS" ]; then
+    return
+  fi
   for group in "${INSTANCE_GROUPS[@]}"; do
     local creation_timestamp=$(gcloud compute instance-groups managed describe \
                               "${group}" \