GoRouter extra is NULL when pressing browser's back button

### What package does this bug report belong to?

go_router

### What target platforms are you seeing this bug on?

Web

### Have you already upgraded your packages?

Yes

### Dependency versions

pubspec.lock

```lock
# Generated by pub
# See https://dart.dev/tools/pub/glossary#lockfile
packages:
async:
dependency: transitive
description:
name: async
sha256: "947bfcf187f74dbc5e146c9eb9c0f10c9f8b30743e341481c1e2ed3ecc18c20c"
url: "https://pub.dev"
source: hosted
version: "2.11.0"
boolean_selector:
dependency: transitive
description:
name: boolean_selector
sha256: "6cfb5af12253eaf2b368f07bacc5a80d1301a071c73360d746b7f2e32d762c66"
url: "https://pub.dev"
source: hosted
version: "2.1.1"
characters:
dependency: transitive
description:
name: characters
sha256: "04a925763edad70e8443c99234dc3328f442e811f1d8fd1a72f1c8ad0f69a605"
url: "https://pub.dev"
source: hosted
version: "1.3.0"
clock:
dependency: transitive
description:
name: clock
sha256: cb6d7f03e1de671e34607e909a7213e31d7752be4fb66a86d29fe1eb14bfb5cf
url: "https://pub.dev"
source: hosted
version: "1.1.1"
collection:
dependency: transitive
description:
name: collection
sha256: ee67cb0715911d28db6bf4af1026078bd6f0128b07a5f66fb2ed94ec6783c09a
url: "https://pub.dev"
source: hosted
version: "1.18.0"
fake_async:
dependency: transitive
description:
name: fake_async
sha256: "511392330127add0b769b75a987850d136345d9227c6b94c96a04cf4a391bf78"
url: "https://pub.dev"
source: hosted
version: "1.3.1"
flutter:
dependency: "direct main"
description: flutter
source: sdk
version: "0.0.0"
flutter_lints:
dependency: "direct dev"
description:
name: flutter_lints
sha256: e2a421b7e59244faef694ba7b30562e489c2b489866e505074eb005cd7060db7
url: "https://pub.dev"
source: hosted
version: "3.0.1"
flutter_test:
dependency: "direct dev"
description: flutter
source: sdk
version: "0.0.0"
flutter_web_plugins:
dependency: transitive
description: flutter
source: sdk
version: "0.0.0"
go_router:
dependency: "direct main"
description:
name: go_router
sha256: "7ecb2f391edbca5473db591b48555a8912dde60edd0fb3013bd6743033b2d3f8"
url: "https://pub.dev"
source: hosted
version: "13.2.1"
leak_tracker:
dependency: transitive
description:
name: leak_tracker
sha256: "78eb209deea09858f5269f5a5b02be4049535f568c07b275096836f01ea323fa"
url: "https://pub.dev"
source: hosted
version: "10.0.0"
leak_tracker_flutter_testing:
dependency: transitive
description:
name: leak_tracker_flutter_testing
sha256: b46c5e37c19120a8a01918cfaf293547f47269f7cb4b0058f21531c2465d6ef0
url: "https://pub.dev"
source: hosted
version: "2.0.1"
leak_tracker_testing:
dependency: transitive
description:
name: leak_tracker_testing
sha256: a597f72a664dbd293f3bfc51f9ba69816f84dcd403cdac7066cb3f6003f3ab47
url: "https://pub.dev"
source: hosted
version: "2.0.1"
lints:
dependency: transitive
description:
name: lints
sha256: cbf8d4b858bb0134ef3ef87841abdf8d63bfc255c266b7bf6b39daa1085c4290
url: "https://pub.dev"
source: hosted
version: "3.0.0"
logging:
dependency: transitive
description:
name: logging
sha256: "623a88c9594aa774443aa3eb2d41807a48486b5613e67599fb4c41c0ad47c340"
url: "https://pub.dev"
source: hosted
version: "1.2.0"
matcher:
dependency: transitive
description:
name: matcher
sha256: d2323aa2060500f906aa31a895b4030b6da3ebdcc5619d14ce1aada65cd161cb
url: "https://pub.dev"
source: hosted
version: "0.12.16+1"
material_color_utilities:
dependency: transitive
description:
name: material_color_utilities
sha256: "0e0a020085b65b6083975e499759762399b4475f766c21668c4ecca34ea74e5a"
url: "https://pub.dev"
source: hosted
version: "0.8.0"
meta:
dependency: transitive
description:
name: meta
sha256: d584fa6707a52763a52446f02cc621b077888fb63b93bbcb1143a7be5a0c0c04
url: "https://pub.dev"
source: hosted
version: "1.11.0"
path:
dependency: transitive
description:
name: path
sha256: "087ce49c3f0dc39180befefc60fdb4acd8f8620e5682fe2476afd0b3688bb4af"
url: "https://pub.dev"
source: hosted
version: "1.9.0"
sky_engine:
dependency: transitive
description: flutter
source: sdk
version: "0.0.99"
source_span:
dependency: transitive
description:
name: source_span
sha256: "53e943d4206a5e30df338fd4c6e7a077e02254531b138a15aec3bd143c1a8b3c"
url: "https://pub.dev"
source: hosted
version: "1.10.0"
stack_trace:
dependency: transitive
description:
name: stack_trace
sha256: "73713990125a6d93122541237550ee3352a2d84baad52d375a4cad2eb9b7ce0b"
url: "https://pub.dev"
source: hosted
version: "1.11.1"
stream_channel:
dependency: transitive
description:
name: stream_channel
sha256: ba2aa5d8cc609d96bbb2899c28934f9e1af5cddbd60a827822ea467161eb54e7
url: "https://pub.dev"
source: hosted
version: "2.1.2"
string_scanner:
dependency: transitive
description:
name: string_scanner
sha256: "556692adab6cfa87322a115640c11f13cb77b3f076ddcc5d6ae3c20242bedcde"
url: "https://pub.dev"
source: hosted
version: "1.2.0"
term_glyph:
dependency: transitive
description:
name: term_glyph
sha256: a29248a84fbb7c79282b40b8c72a1209db169a2e0542bce341da992fe1bc7e84
url: "https://pub.dev"
source: hosted
version: "1.2.1"
test_api:
dependency: transitive
description:
name: test_api
sha256: "5c2f730018264d276c20e4f1503fd1308dfbbae39ec8ee63c5236311ac06954b"
url: "https://pub.dev"
source: hosted
version: "0.6.1"
vector_math:
dependency: transitive
description:
name: vector_math
sha256: "80b3257d1492ce4d091729e3a67a60407d227c27241d6927be0130c98e741803"
url: "https://pub.dev"
source: hosted
version: "2.1.4"
vm_service:
dependency: transitive
description:
name: vm_service
sha256: b3d56ff4341b8f182b96aceb2fa20e3dcb336b9f867bc0eafc0de10f1048e957
url: "https://pub.dev"
source: hosted
version: "13.0.0"
sdks:
dart: ">=3.3.0 <4.0.0"
flutter: ">=3.16.0"

```

### Steps to reproduce

Using the sample code below:
1. Press `Hello World!` to navigate to `SettingsPage`.
2. In `SettingsPage`, press `Hello from Home`. This will push another `SettingsPage`.
3. In the 2nd `SettingsPage`, press browser's back button.

### Expected results

After pressing browser's back button, 1st `SettingsPage`'s extra should not be NULL.

### Actual results

After pressing browser's back button, 1st `SettingsPage`'s extra is NULL.

However, if I set the extra to string, e.g. `extra: 'from Home',`, it's not cleared. Wondering why class extra is being cleared.

### Code sample

Code sample

```dart
import 'package:flutter/material.dart';
import 'package:go_router/go_router.dart';

void main() {
GoRouter.optionURLReflectsImperativeAPIs = true;
runApp(const MainApp());
}

class MainApp extends StatelessWidget {
const MainApp({
super.key,
});

@override
Widget build(BuildContext context) {
return MaterialApp.router(
routerConfig: GoRouter(
initialLocation: '/',
routes: [
GoRoute(
path: '/',
builder: (_, __) => const HomePage(),
),
GoRoute(
path: '/settings',
builder: (_, state) {
print('>>>>> settings extra = ${state.extra}');

return SettingsPage(
extra: switch (state.extra) {
SettingsPageParams(message: final message) => message,
_ => state.extra?.toString(),
},
);
},
),
],
),
);
}
}

//--------------------------------------------------------------------------------------------------
class HomePage extends StatelessWidget {
const HomePage({
super.key,
});

@override
Widget build(BuildContext context) {
return Scaffold(
body: Center(
child: TextButton(
onPressed: () => context.push(
'/settings',
extra: const SettingsPageParams('from Home'),
),
child: const Text('Hello World!'),
),
),
);
}
}

//--------------------------------------------------------------------------------------------------
class SettingsPage extends StatelessWidget {
const SettingsPage({
this.extra,
super.key,
});

final String? extra;

@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(),
body: Center(
child: TextButton(
onPressed: () => context.push(
'/settings',
extra: const SettingsPageParams('from Settings'),
),
child: Text('Hello $extra'),
),
),
);
}
}

//--------------------------------------------------------------------------------------------------
class SettingsPageParams {
const SettingsPageParams(this.message);

final String message;
}
```

### Screenshots or Videos

Screenshots / Video demonstration

Class as extra.
https://github.com/flutter/flutter/assets/148017560/2861ba03-2822-4faf-97f6-2d08286deb4a

String as extra.
https://github.com/flutter/flutter/assets/148017560/7535ebaf-1a19-4c80-a41d-a148709c67c2

### Logs

Logs

```console
[Paste your logs here]
```

### Flutter Doctor output

Doctor output

```console
[✓] Flutter (Channel stable, 3.19.3, on macOS 14.4 23E214 darwin-arm64, locale en-PH)
• Flutter version 3.19.3 on channel stable at /Users/rickimaru/Documents/GitHub/flutter
• Upstream repository https://github.com/flutter/flutter.git
• Framework revision ba39319843 (13 days ago), 2024-03-07 15:22:21 -0600
• Engine revision 2e4ba9c6fb
• Dart version 3.3.1
• DevTools version 2.31.1

[✓] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
• Android SDK at /Users/rickimaru/Library/Android/sdk
• Platform android-34, build-tools 34.0.0
• Java binary at: /Applications/Android Studio.app/Contents/jbr/Contents/Home/bin/java
• Java version OpenJDK Runtime Environment (build 17.0.6+0-17.0.6b829.9-10027231)
• All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 15.3)
• Xcode at /Applications/Xcode.app/Contents/Developer
• Build 15E204a
• CocoaPods version 1.13.0

[✓] Chrome - develop for the web
• CHROME_EXECUTABLE = /Applications/Brave Browser.app/Contents/MacOS/Brave Browser

[✓] Android Studio (version 2022.3)
• Android Studio at /Applications/Android Studio.app/Contents
• Flutter plugin can be installed from:
🔨 https://plugins.jetbrains.com/plugin/9212-flutter
• Dart plugin can be installed from:
🔨 https://plugins.jetbrains.com/plugin/6351-dart
• Java version OpenJDK Runtime Environment (build 17.0.6+0-17.0.6b829.9-10027231)

[✓] VS Code (version 1.87.2)
• VS Code at /Applications/Visual Studio Code.app/Contents
• Flutter extension version 3.84.0

[✓] Connected device (2 available)
• macOS (desktop) • macos • darwin-arm64 • macOS 14.4 23E214 darwin-arm64
• Chrome (web) • chrome • web-javascript • Brave Browser 123.1.64.109

[✓] Network resources
• All expected network resources are available.

• No issues found!
```