[SPARK-13441][YARN] Fix NPE in yarn Client.createConfArchive method

## What changes were proposed in this pull request?

Instead of using result of File.listFiles() directly, which may throw NPE, check for null first. If it is null, log a warning instead

## How was the this patch tested?

Ran the ./dev/run-tests locally
Tested manually on a cluster

Author: Terence Yim <terence@cask.co>

Closes #11337 from chtyim/fixes/SPARK-13441-null-check.

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

@@ -537,9 +537,14 @@ private[spark] class Client(
       sys.env.get(envKey).foreach { path =>
         val dir = new File(path)
         if (dir.isDirectory()) {
-          dir.listFiles().foreach { file =>
-            if (file.isFile && !hadoopConfFiles.contains(file.getName())) {
-              hadoopConfFiles(file.getName()) = file
+          val files = dir.listFiles()
+          if (files == null) {
+            logWarning("Failed to list files under directory " + dir)
+          } else {
+            files.foreach { file =>
+              if (file.isFile && !hadoopConfFiles.contains(file.getName())) {
+                hadoopConfFiles(file.getName()) = file
+              }
             }
           }
         }