[webview_flutter_wkwebview] Fixes bug where WebkitWebViewController.getUserAgent was incorrectly returning an empty String (flutter#5062)

It seems that [WKWebView.customUserAgent](https://developer.apple.com/documentation/webkit/wkwebview/1414950-customuseragent?language=objc) is never actually set to nil. It was always an empty String. Even after attempting to set the value to nil, the value would always return as an empty String. This is in contrary to the documentation for the property.

I'm not sure if this is based on the iOS version, so this PR checks if the value is null or is empty.

Fixes flutter#135813

Author: bparrishMines

packages/webview_flutter/webview_flutter_wkwebview/CHANGELOG.md

@@ -1,3 +1,7 @@
+## 3.9.1
+
+* Fixes bug where `WebkitWebViewController.getUserAgent` was incorrectly returning an empty String.
+
 ## 3.9.0
 
 * Adds support for `PlatformWebViewController.getUserAgent`.

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

@@ -313,6 +313,7 @@ Future<void> main() async {
 
     final String? userAgent = await controller.getUserAgent();
     expect(userAgent, isNotNull);
+    expect(userAgent, isNotEmpty);
   });
 
   group('Video playback policy', () {

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

@@ -666,7 +666,10 @@ window.addEventListener("error", function(e) {
   @override
   Future<String?> getUserAgent() async {
     final String? customUserAgent = await _webView.getCustomUserAgent();
-    if (customUserAgent != null) {
+    // Despite the official documentation of `WKWebView.customUserAgent`, the
+    // default value seems to be an empty String and not null. It's possible it
+    // could depend on the iOS version, so this checks for both.
+    if (customUserAgent != null && customUserAgent.isNotEmpty) {
       return customUserAgent;
     }
 

packages/webview_flutter/webview_flutter_wkwebview/pubspec.yaml

@@ -2,7 +2,7 @@ name: webview_flutter_wkwebview
 description: A Flutter plugin that provides a WebView widget based on Apple's WKWebView control.
 repository: https://github.com/flutter/packages/tree/main/packages/webview_flutter/webview_flutter_wkwebview
 issue_tracker: https://github.com/flutter/flutter/issues?q=is%3Aissue+is%3Aopen+label%3A%22p%3A+webview%22
-version: 3.9.0
+version: 3.9.1
 
 environment:
   sdk: ">=2.19.0 <4.0.0"