Fix `DataTable`'s `headingTextStyle` & `dataTextStyle` are not merged with default text style (#134138)

fixes [Inconsistent text color on DataTable in different platforms](https://github.com/flutter/flutter/issues/114470)

### Code sample

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

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

/// Flutter code sample for [DataTable].

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

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      themeMode: ThemeMode.dark,
      theme: ThemeData(),
      darkTheme: ThemeData.dark(),
      home: Scaffold(
        appBar: AppBar(title: const Text('DataTable Sample')),
        body: const DataTableExample(),
      ),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return DataTable(
      headingTextStyle: const TextStyle(),
      dataTextStyle: const TextStyle(),
      columns: const <DataColumn>[
        DataColumn(
          label: Expanded(
            child: Text(
              'Name',
              style: TextStyle(fontStyle: FontStyle.italic),
            ),
          ),
        ),
        DataColumn(
          label: Expanded(
            child: Text(
              'Age',
              style: TextStyle(fontStyle: FontStyle.italic),
            ),
          ),
        ),
        DataColumn(
          label: Expanded(
            child: Text(
              'Role',
              style: TextStyle(fontStyle: FontStyle.italic),
            ),
          ),
        ),
      ],
      rows: const <DataRow>[
        DataRow(
          cells: <DataCell>[
            DataCell(Text('Sarah')),
            DataCell(Text('19')),
            DataCell(Text('Student')),
          ],
        ),
        DataRow(
          cells: <DataCell>[
            DataCell(Text('Janine')),
            DataCell(Text('43')),
            DataCell(Text('Professor')),
          ],
        ),
        DataRow(
          cells: <DataCell>[
            DataCell(Text('William')),
            DataCell(Text('27')),
            DataCell(Text('Associate Professor')),
          ],
        ),
      ],
    );
  }
}

```

</details>

### Before

| Desktop | Mobile |
| --------------- | --------------- |
| <img src="https://github.com/flutter/flutter/assets/48603081/19c3908d-6b6a-4408-9c6b-da83c8efaa4a"  /> | <img src="https://github.com/flutter/flutter/assets/48603081/efda08fb-05f9-437e-be5c-6b6861babe19" width="350"  /> |

### After

| Desktop | Mobile |
| --------------- | --------------- |
| <img src="https://github.com/flutter/flutter/assets/48603081/6bd3433f-d61f-4f35-8a2a-f7539a74f93e" /> | <img src="https://github.com/flutter/flutter/assets/48603081/5123a79b-6c2a-4bea-9fbc-64ed3e599826" width="350" /> |