Return '(null)' when message.body is null

packages/webview_flutter/webview_flutter_wkwebview/example/integration_test/webview_flutter_test.dart

@@ -316,7 +316,7 @@ Future<void> main() async {
     await pageFinished.future;
 
     await controller.runJavaScript('Channel.postMessage(undefined);');
-    await expectLater(channelCompleter.future, completion(''));
+    await expectLater(channelCompleter.future, completion('(null)'));
   });
 
   testWidgets('resize webview', (WidgetTester tester) async {

packages/webview_flutter/webview_flutter_wkwebview/lib/src/webkit_webview_controller.dart

@@ -905,8 +905,13 @@ class WebKitJavaScriptChannelParams extends JavaScriptChannelParams {
             (WeakReference<void Function(JavaScriptMessage)> weakReference) {
               return (_, __, WKScriptMessage message) {
                 if (weakReference.target != null) {
+                  // When message.body is null, return '(null)' for consistency
+                  // with previous implementations.
                   weakReference.target!(
-                    JavaScriptMessage(message: message.body?.toString() ?? ''),
+                    JavaScriptMessage(
+                        message: message.body == null
+                            ? '(null)'
+                            : message.body!.toString()),
                   );
                 }
               };