YARN-10823. Fix default node label if no label is defined

Author: 9uapaw

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/capacity/ConfiguredNodeLabels.java

@@ -81,7 +81,13 @@ public void setLabelsByQueue(
    * @return all node labels
    */
   public Set<String> getAllConfiguredLabels() {
-    return configuredNodeLabelsByQueue.values().stream()
+    Set<String> nodeLabels = configuredNodeLabelsByQueue.values().stream()
         .flatMap(Set::stream).collect(Collectors.toSet());
+
+    if (nodeLabels.size() == 0) {
+      nodeLabels = NO_LABEL;
+    }
+
+    return nodeLabels;
   }
 }