[SPARK-20354]/api/v1/applications’ return sparkUser is null in REST API.

Author: guoxiaolong

core/src/main/scala/org/apache/spark/ui/SparkUI.scala

@@ -117,7 +117,7 @@ private[spark] class SparkUI private (
         endTime = new Date(-1),
         duration = 0,
         lastUpdated = new Date(startTime),
-        sparkUser = "",
+        sparkUser = getSparkUser,
         completed = false
       ))
     ))