YARN-10934. Fix LeafQueue#activateApplication NPE when the user of the pending application is missing from usersManager. Contributed by Benjamin Teke

Co-authored-by: Benjamin Teke <bteke@cloudera.com>

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

@@ -878,7 +878,7 @@ public class LeafQueue extends AbstractCSQueue {
         }
 
         // Check user am resource limit
-        User user = getUser(application.getUser());
+        User user = usersManager.getUserAndAddIfAbsent(application.getUser());
         Resource userAMLimit = userAmPartitionLimit.get(partitionName);
 
         // Verify whether we already calculated user-am-limit for this label.