YARN-5754. Null check missing for earliest in FifoPolicy. (Yufei Gu via kasha)

(cherry picked from commit a71fc81655cd5382d354674dd06570ba49753688)

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-server/hadoop-yarn-server-resourcemanager/src/main/java/org/apache/hadoop/yarn/server/resourcemanager/scheduler/fair/policies/FifoPolicy.java

@@ -96,7 +96,10 @@ public class FifoPolicy extends SchedulingPolicy {
         earliest = schedulable;
       }
     }
-    earliest.setFairShare(Resources.clone(totalResources));
+
+    if (earliest != null) {
+      earliest.setFairShare(Resources.clone(totalResources));
+    }
   }
 
   @Override