YARN-9749. TestAppLogAggregatorImpl#testDFSQuotaExceeded fails on trunk. Contributed by Adam Antal

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-nodemanager/src/main/java/org/apache/hadoop/yarn/server/nodemanager/containermanager/logaggregation/AppLogAggregatorImpl.java

@@ -395,7 +395,7 @@ public class AppLogAggregatorImpl implements AppLogAggregator {
       if (logAggregationSucceedInThisCycle && deletionTask != null) {
         delService.delete(deletionTask);
       }
-      if (!diagnosticMessage.isEmpty()) {
+      if (diagnosticMessage != null && !diagnosticMessage.isEmpty()) {
         LOG.debug("Sending log aggregation report along with the " +
             "following diagnostic message:\"{}\"", diagnosticMessage);
       }