Protect from setting null environment variables.

core/src/main/scala/spark/deploy/worker/ExecutorRunner.scala

@@ -116,7 +116,11 @@ private[spark] class ExecutorRunner(
       val builder = new ProcessBuilder(command: _*).directory(executorDir)
       val env = builder.environment()
       for ((key, value) <- jobDesc.command.environment) {
-        env.put(key, value)
+        if (value == null) {
+          logInfo("Environment variable not set: " + key)
+        } else {
+          env.put(key, value)
+        }
       }
       // In case we are running this from within the Spark Shell
       // so we are not creating a parent process.