[stream] Readable.redableFlowing should be nullary

app/nodejs-v12/src/test/scala/io/scalajs/nodejs/stream/ReadableTest.scala

@@ -8,5 +8,12 @@ class ReadableTest extends AnyFunSpec {
     it("readableEncoding") {
       assert(new ReadStream("package.json").readableEncoding === null)
     }
+
+    it("readableFlowing") {
+      val rs = new ReadStream("package.json")
+      assert(rs.readableFlowing === null)
+      rs.on("readable", () => {})
+      assert(rs.readableFlowing === false)
+    }
   }
 }

app/nodejs-v14/src/main/scala/io/scalajs/nodejs/stream/Stream.scala

@@ -159,7 +159,7 @@ sealed trait IReadable extends LegacyStream {
   @enableIf(io.scalajs.nodejs.internal.CompilerSwitches.gteNodeJs12)
   def readableEnded: Boolean = js.native
 
-  def readableFlowing: Boolean = js.native
+  def readableFlowing: Boolean | Null = js.native
 
   def readableHighWaterMark: Double = js.native