Properly handle no extra python files.

Puts in a filler for the environment variable so that the arguments are
passed through correctly.

Author: mccheah

resource-managers/kubernetes/core/src/main/scala/org/apache/spark/deploy/kubernetes/submit/submitsteps/PythonStep.scala

@@ -27,15 +27,19 @@ private[spark] class PythonStep(
     filesDownloadPath: String) extends KubernetesSubmissionStep {
 
   override def prepareSubmission(driverSpec: KubernetesDriverSpec): KubernetesDriverSpec = {
+    val resolvedOtherPyFilesString = if (otherPyFiles.isEmpty) {
+      "no-py-files"
+    } else {
+      otherPyFiles.mkString(",")
+    }
     val withPythonPrimaryFileContainer = new ContainerBuilder(driverSpec.driverContainer)
       .addNewEnv()
         .withName(ENV_PYSPARK_PRIMARY)
         .withValue(KubernetesFileUtils.resolveFilePath(primaryPyFile, filesDownloadPath))
         .endEnv()
       .addNewEnv()
         .withName(ENV_PYSPARK_FILES)
-        .withValue(
-            KubernetesFileUtils.resolveFilePaths(otherPyFiles, filesDownloadPath).mkString(","))
+        .withValue(resolvedOtherPyFilesString)
         .endEnv()
     driverSpec.copy(driverContainer = withPythonPrimaryFileContainer.build())
   }