title: Java/Kotlin- Akka Stream Source.reduce does not work if null in Source
body: <p>In the Akka Streams Doku, a Scala example with a list containing a null is given. This list transformed into a Source, and reduced like in the example. Unfortinutly, in Java/Kotlin this does not output anything.</p>
  
  <p>Link to Scala example with tweets: <a href="https://doc.akka.io/docs/akka/current/stream/stream-quickstart.html#first-steps" rel="nofollow noreferrer">https://doc.akka.io/docs/akka/current/stream/stream-quickstart.html#first-steps</a></p>
  
  <p>Here my translation into Kotlin</p>
  
  <pre><code>val system = ActorSystem.create("reactive-tweets")
  
  val ints: Source&lt;Int, NotUsed&gt; = Source.from(listOf(1, 2, 3, 4, 5, 6, 7, 8, 9, 10, null))
  ints
    .filter {
      it != null
    }
    .map {
      it * 2
    }
    .reduce { arg1, arg2 -&gt;
      arg1 + arg2
    }
    .runWith(Sink.foreach { a -&gt; println("sum: " + a)}, system)
  </code></pre>
  
  <p>I also tried the <code>Source&lt;Int?, NotUsed&gt;</code> and didn't change anything.
  I would love to know if in the backend an error occurs or what happens, that the stream does not reach the print statement.</p>