fixes [NavigationDrawer selected item has wrong icon color [Material3 spec]](https://github.com/flutter/flutter/issues/129572)

### Description
This PR fixes a mistake in the `NavigationDrawer` defaults, where generated token value returns a `null`. 
This issue can be detected when you want to customize the selected icon color for `NavigationDrawerDestination` using a custom color scheme.

### Code sample

<details> 
<summary>expanded to view the code sample</summary> 

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

void main() => runApp(const MyApp());

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      debugShowCheckedModeBanner: false,
      themeMode: ThemeMode.light,
      theme: ThemeData(
        colorScheme: ColorScheme.fromSeed(seedColor: Colors.blue).copyWith(
          onSecondaryContainer: Colors.red,
        ),
        useMaterial3: true,
      ),
      home: const Example(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: const Text('NavigationDrawer Sample'),
      ),
      drawer: const NavigationDrawer(
        children: <Widget>[
          NavigationDrawerDestination(
            icon: Icon(Icons.favorite_outline_rounded),
            label: Text('Favorite'),
            selectedIcon: Icon(Icons.favorite_rounded),
          ),
          NavigationDrawerDestination(
            icon: Icon(Icons.favorite_outline_rounded),
            label: Text('Favorite'),
          ),
        ],
      ),
    );
  }
}
``` 
	
</details>

### Before
 
<img width="1053" alt="Screenshot 2023-06-27 at 13 24 38" src="https://github.com/flutter/flutter/assets/48603081/18c13a73-688f-4586-bb60-bddef45d173f">

### After

<img width="1053" alt="Screenshot 2023-06-27 at 13 24 25" src="https://github.com/flutter/flutter/assets/48603081/8a1427c6-517f-424a-b0bd-24bad7c5fbb0">