YARN-10703. Fix potential null pointer error of gpuNodeResourceUpdateHandler in NodeResourceMonitorImpl. Contributed by Qi Zhu.

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-nodemanager/src/main/java/org/apache/hadoop/yarn/server/nodemanager/NodeResourceMonitorImpl.java

@@ -173,8 +173,10 @@ public class NodeResourceMonitorImpl extends AbstractService implements
 
         float nodeGpuUtilization = 0F;
         try {
-          nodeGpuUtilization =
-              gpuNodeResourceUpdateHandler.getNodeGpuUtilization();
+          if (gpuNodeResourceUpdateHandler != null) {
+            nodeGpuUtilization =
+                gpuNodeResourceUpdateHandler.getNodeGpuUtilization();
+          }
         } catch (Exception e) {
           LOG.error("Get Node GPU Utilization error: " + e);
         }