add Source.never (#29008)

* add Source.never

* make Source.never single instance, add DefaultAttributes

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

@@ -304,6 +304,13 @@ object Source {
   def future[T](futureElement: Future[T]): Source[T, NotUsed] =
     scaladsl.Source.future(futureElement).asJava
 
+  /**
+   * Never emits any elements, never completes and never fails.
+   * This stream could be useful in tests.
+   */
+  def never[T]: Source[T, NotUsed] =
+    scaladsl.Source.never.asJava
+
   /**
    * Emits a single value when the given `CompletionStage` is successfully completed and then completes the stream.
    * If the `CompletionStage` is completed with a failure the stream is failed.