YARN-11764. Add the JsonInclude annotation.

Author: fanshilun

hadoop-yarn-project/hadoop-yarn/hadoop-yarn-api/src/main/java/org/apache/hadoop/yarn/api/records/timelineservice/ApplicationEntity.java

@@ -17,6 +17,7 @@
  */
 package org.apache.hadoop.yarn.api.records.timelineservice;
 
+import com.fasterxml.jackson.annotation.JsonInclude;
 import org.apache.hadoop.classification.InterfaceAudience;
 import org.apache.hadoop.classification.InterfaceStability;
 
@@ -42,13 +43,12 @@ public ApplicationEntity(TimelineEntity entity) {
     }
   }
 
+  @JsonInclude(JsonInclude.Include.NON_NULL)
   public String getQueue() {
-    if (getInfo() != null) {
-      if (getInfo().containsKey(QUEUE_INFO_KEY)) {
-        return getInfo().get(QUEUE_INFO_KEY).toString();
-      }
+    if (getInfo() != null && getInfo().containsKey(QUEUE_INFO_KEY)) {
+      return getInfo().get(QUEUE_INFO_KEY).toString();
     }
-    return "";
+    return null;
   }
 
   public void setQueue(String queue) {