Fix RawChip doesn't use ChipTheme.showCheckmark value (#131257)
fixes [`RawChip` doesn't use `ChipThemeData.showCheckmark` value](https://github.com/flutter/flutter/issues/119163)
### Description
`RawChip.showCheckmark` is nullable yet the constructor falsely assigns a default which breaks `ChipTheme` support. This PR removes the falsely assigned default value.
### Code sample
<details>
<summary>expand 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,
theme: ThemeData(useMaterial3: true,
chipTheme: const ChipThemeData(
showCheckmark: false,
)
),
home: const Example(),
);
}
}
class Example extends StatelessWidget {
const Example({super.key});
@override
Widget build(BuildContext context) {
return Scaffold(
appBar: AppBar(
title: const Text('Sample'),
),
body: Center(
child: Row(
mainAxisAlignment: MainAxisAlignment.spaceEvenly,
children: <Widget>[
const RawChip(
selected: true,
label: Text('RawChip'),
),
FilterChip(
selected: true,
label: const Text('RawChip'), onSelected: (bool value) { },
),
],
),
),
);
}
}
```
</details>
### Before
### After
62adfcf737