YARN-8068. Application Priority field causes NPE in app timeline publish when Hadoop 2.7 based clients to 2.8+ (Sunil G via wangda)

Change-Id: I7910bd1064a1b4dbbe2084080c060822ea6f3b48
(cherry picked from commit 9eef19b2ad78b8464da252d0e23c08675898b9d8)
(cherry picked from commit 62b95373329c64b90d266019f98f423f3c635351)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/rmapp/RMAppImpl.java

@@ -455,6 +455,11 @@ public class RMAppImpl implements RMApp, Recoverable {
     if (submissionContext.getPriority() != null) {
       this.applicationPriority = Priority
           .newInstance(submissionContext.getPriority().getPriority());
+    } else {
+      // If incoming app does not have priority configured in submission
+      // context, system could be assume that its a 0 priority app and could be
+      // considered as normal.
+      this.applicationPriority = Priority.newInstance(0);
     }
 
     int globalMaxAppAttempts = conf.getInt(YarnConfiguration.RM_AM_MAX_ATTEMPTS,