Don't include the empty string "" as a defaultAclUser

Author: ash211

core/src/main/scala/org/apache/spark/SecurityManager.scala

@@ -162,7 +162,7 @@ private[spark] class SecurityManager(sparkConf: SparkConf) extends Logging {
 
   // always add the current user and SPARK_USER to the viewAcls
   private val defaultAclUsers = Set[String](System.getProperty("user.name", ""),
-    Option(System.getenv("SPARK_USER")).getOrElse(""))
+    Option(System.getenv("SPARK_USER")).getOrElse("")).filter(_ != "")
 
   setViewAcls(defaultAclUsers, sparkConf.get("spark.ui.view.acls", ""))
   setModifyAcls(defaultAclUsers, sparkConf.get("spark.modify.acls", ""))