Fix task metrics NullPointerException in KafkaSinkExtended

[grihabor]

Fixes the following error in KafkaSinkExtended:

java.lang.NullPointerException: Cannot invoke "org.apache.spark.executor.TaskMetrics.executorRunTime()" because "taskmetrics" is null
        at ch.cern.sparkmeasure.KafkaSinkExtended.onTaskEnd(KafkaSink.scala:317)
        at org.apache.spark.scheduler.SparkListenerBus.doPostEvent(SparkListenerBus.scala:45)
        at org.apache.spark.scheduler.SparkListenerBus.doPostEvent$(SparkListenerBus.scala:28)
        at org.apache.spark.scheduler.AsyncEventQueue.doPostEvent(AsyncEventQueue.scala:37)
        at org.apache.spark.scheduler.AsyncEventQueue.doPostEvent(AsyncEventQueue.scala:37)
        at org.apache.spark.util.ListenerBus.postToAll(ListenerBus.scala:117)
        at org.apache.spark.util.ListenerBus.postToAll$(ListenerBus.scala:101)
        at org.apache.spark.scheduler.AsyncEventQueue.super$postToAll(AsyncEventQueue.scala:105)
        at org.apache.spark.scheduler.AsyncEventQueue.$anonfun$dispatch$1(AsyncEventQueue.scala:105)
        at scala.runtime.java8.JFunction0$mcJ$sp.apply(JFunction0$mcJ$sp.java:23)
        at scala.util.DynamicVariable.withValue(DynamicVariable.scala:62)
        at org.apache.spark.scheduler.AsyncEventQueue.org$apache$spark$scheduler$AsyncEventQueue$$dispatch(AsyncEventQueue.scala:100)
        at org.apache.spark.scheduler.AsyncEventQueue$$anon$2.$anonfun$run$1(AsyncEventQueue.scala:96)
        at org.apache.spark.util.Utils$.tryOrStopSparkContext(Utils.scala:1356)
        at org.apache.spark.scheduler.AsyncEventQueue$$anon$2.run(AsyncEventQueue.scala:96)

[grihabor]

Hey @LucaCanali , thank you for your project!

Do you have an estimate for 0.27 release by any chance? Thank you