str #20262 reduce should fail explicitly on empty stream (#20267)

* str #20262 reduce should fail explicitly on empty stream

* str #20262 document reduce behaviour on empty stream

akka-stream/src/main/scala/akka/stream/scaladsl/Source.scala

@@ -107,6 +107,11 @@ final class Source[+Out, +Mat](private[stream] override val module: Module)
    * The returned [[scala.concurrent.Future]] will be completed with value of the final
    * function evaluation when the input stream ends, or completed with `Failure`
    * if there is a failure signaled in the stream.
+   *
+   * If the stream is empty (i.e. completes before signalling any elements),
+   * the reduce stage will fail its downstream with a [[NoSuchElementException]],
+   * which is semantically in-line with that Scala's standard library collections
+   * do in such situations.
    */
   def runReduce[U >: Out](f: (U, U) ⇒ U)(implicit materializer: Materializer): Future[U] =
     runWith(Sink.reduce(f))