chore: Add the missing EmptySource case to TraversalBuilder (#1743)

Author: He-Pin

stream-tests/src/test/scala/org/apache/pekko/stream/impl/TraversalBuilderSpec.scala

@@ -17,7 +17,7 @@ import org.apache.pekko
 import pekko.NotUsed
 import pekko.stream._
 import pekko.stream.impl.TraversalTestUtils._
-import pekko.stream.scaladsl.Keep
+import pekko.stream.scaladsl.{ Keep, Source }
 import pekko.testkit.PekkoSpec
 
 class TraversalBuilderSpec extends PekkoSpec {
@@ -447,4 +447,20 @@ class TraversalBuilderSpec extends PekkoSpec {
     }
   }
 
+  "find Source.empty via TraversalBuilder with isEmptySource" in {
+    val emptySource = EmptySource
+    TraversalBuilder.isEmptySource(emptySource) should be(true)
+  }
+
+  "find javadsl Source.empty via TraversalBuilder with isEmptySource" in {
+    import pekko.stream.javadsl.Source
+    val emptySource = Source.empty()
+    TraversalBuilder.isEmptySource(emptySource) should be(true)
+  }
+
+  "find scaldsl Source.empty via TraversalBuilder with isEmptySource" in {
+    val emptySource = Source.empty
+    TraversalBuilder.isEmptySource(emptySource) should be(true)
+  }
+
 }

stream/src/main/scala/org/apache/pekko/stream/impl/TraversalBuilder.scala

@@ -386,6 +386,7 @@ import pekko.util.unused
   def isEmptySource(graph: Graph[SourceShape[_], _]): Boolean = graph match {
     case source: scaladsl.Source[_, _] if source eq scaladsl.Source.empty => true
     case source: javadsl.Source[_, _] if source eq javadsl.Source.empty() => true
+    case EmptySource                                                      => true
     case _                                                                => false
   }