Fix RawChip doesn't use ChipTheme.showCheckmark value (flutter#131257)

fixes [`RawChip` doesn't use `ChipThemeData.showCheckmark` value](flutter#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
![before](https://github.com/flutter/flutter/assets/48603081/c8050c28-d988-4c72-8e0a-6455aa02d119)

### After

![after](https://github.com/flutter/flutter/assets/48603081/d5e83e81-6c12-4594-a2fd-8f113d6c9b54)

Author: TahaTesser

packages/flutter/lib/src/material/chip.dart

@@ -748,7 +748,7 @@ class RawChip extends StatefulWidget
     this.surfaceTintColor,
     this.iconTheme,
     this.selectedShadowColor,
-    this.showCheckmark = true,
+    this.showCheckmark,
     this.checkmarkColor,
     this.avatarBorder = const CircleBorder(),
     @Deprecated(

packages/flutter/test/material/chip_theme_test.dart

@@ -71,6 +71,7 @@ void main() {
     expect(themeData.brightness, null);
     expect(themeData.elevation, null);
     expect(themeData.pressElevation, null);
+    expect(themeData.iconTheme, null);
   });
 
   testWidgetsWithLeakTracking('Default ChipThemeData debugFillProperties', (WidgetTester tester) async {
@@ -108,14 +109,15 @@ void main() {
       brightness: Brightness.dark,
       elevation: 5,
       pressElevation: 6,
+      iconTheme: IconThemeData(color: Color(0xffffff10)),
     ).debugFillProperties(builder);
 
     final List<String> description = builder.properties
         .where((DiagnosticsNode node) => !node.isFiltered(DiagnosticLevel.info))
         .map((DiagnosticsNode node) => node.toString())
         .toList();
 
-    expect(description, <String>[
+    expect(description, equalsIgnoringHashCodes(<String>[
       'color: MaterialStatePropertyAll(Color(0xfffffff0))',
       'backgroundColor: Color(0xfffffff1)',
       'deleteIconColor: Color(0xfffffff2)',
@@ -136,7 +138,8 @@ void main() {
       'brightness: dark',
       'elevation: 5.0',
       'pressElevation: 6.0',
-    ]);
+      'iconTheme: IconThemeData#00000(color: Color(0xffffff10))'
+    ]));
   });
 
   testWidgetsWithLeakTracking('Chip uses ThemeData chip theme', (WidgetTester tester) async {
@@ -868,6 +871,63 @@ void main() {
     // Enabled & selected chip should have the provided selectedColor.
     expect(getMaterialBox(tester), paints..rrect(color: chipTheme.selectedColor));
   });
+
+  // This is a regression test for https://github.com/flutter/flutter/issues/119163.
+  testWidgetsWithLeakTracking('RawChip respects checkmark properties from ChipTheme', (WidgetTester tester) async {
+    Widget buildRawChip({ChipThemeData? chipTheme}) {
+      return MaterialApp(
+        theme: ThemeData.light(useMaterial3: false).copyWith(
+          chipTheme: chipTheme,
+        ),
+        home: Directionality(
+          textDirection: TextDirection.ltr,
+          child: Material(
+          child: Center(
+              child: RawChip(
+                selected: true,
+                label: const SizedBox(width: 100, height: 100),
+                onSelected: (bool newValue) { },
+              ),
+            ),
+          ),
+        ),
+      );
+    }
+
+    // Test that the checkmark is painted.
+    await tester.pumpWidget(buildRawChip(
+      chipTheme: const ChipThemeData(
+        checkmarkColor: Color(0xffff0000),
+      ),
+    ));
+
+    RenderBox materialBox = getMaterialBox(tester);
+    expect(
+      materialBox,
+      paints..path(
+        color: const Color(0xffff0000),
+        style: PaintingStyle.stroke,
+      ),
+    );
+
+    // Test that the checkmark is not painted when ChipThemeData.showCheckmark is false.
+    await tester.pumpWidget(buildRawChip(
+      chipTheme: const ChipThemeData(
+        showCheckmark: false,
+        checkmarkColor: Color(0xffff0000),
+      ),
+    ));
+    await tester.pumpAndSettle();
+
+    materialBox = getMaterialBox(tester);
+    expect(
+      materialBox,
+      isNot(paints..path(
+        color: const Color(0xffff0000),
+        style: PaintingStyle.stroke,
+      )),
+    );
+  });
 }
 
 class _MaterialStateOutlinedBorder extends StadiumBorder implements MaterialStateOutlinedBorder {