Guard against potential NPE in debug logging mode

The getClientToken (or getClientToAMToken) method in Hadoop
apparently returns null sometimes. We need to prevent NPEs that
result from this. Yay documentation.

Author: andrewor14

yarn/alpha/src/main/scala/org/apache/spark/deploy/yarn/Client.scala

@@ -122,8 +122,12 @@ private[spark] class Client(
     amContainer.setContainerTokens(ByteBuffer.wrap(dob.getData()))
   }
 
-  /** */
-  override def getClientToken(report: ApplicationReport): String = report.getClientToken
+  /**
+   * Return the security token used by this client to communicate with the ApplicationMaster.
+   * If no security is enabled, the token returned by the report is null.
+   */
+  override def getClientToken(report: ApplicationReport): String =
+    Option(report.getClientToken).getOrElse("")
 }
 
 private[spark] object Client {

yarn/stable/src/main/scala/org/apache/spark/deploy/yarn/Client.scala

@@ -108,10 +108,12 @@ private[spark] class Client(
   override def getApplicationReport(appId: ApplicationId): ApplicationReport =
     yarnClient.getApplicationReport(appId)
 
-  /** */
-  // FIXME: This could throw NPE
+  /**
+   * Return the security token used by this client to communicate with the ApplicationMaster.
+   * If no security is enabled, the token returned by the report is null.
+   */
   override def getClientToken(report: ApplicationReport): String =
-    report.getClientToAMToken.toString
+    Option(report.getClientToAMToken).map(_.toString).getOrElse("")
 }
 
 private[spark] object Client {