Fix DataTable's headingTextStyle & dataTextStyle are not merged with default text style

[TahaTesser]

fixes Inconsistent text color on DataTable in different platforms

Code sample

expand to view the code sample

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')),
          ],
        ),
      ],
    );
  }
}

Before

Desktop	Mobile

After

Desktop	Mobile

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or this PR is test-exempt.
• All existing and new tests are passing.

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[HansMuller]

LGTM

[auto-submit]

auto label is removed for flutter/flutter/134138, due to - The status or check suite Google testing has failed. Please fix the issues identified (or deflake) before re-applying this label.