Fix transparent dividerColor breaks TabBar.tabAlignment (#150350)
fixes [`TabBar.tabAlignment` property does't work when `dividerColor` is transparent](https://github.com/flutter/flutter/issues/150316)
### 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,
home: DefaultTabController(
length: 2,
child: Scaffold(
appBar: AppBar(
title: const Text('TabBar'),
bottom: const TabBar(
dividerColor: Colors.transparent,
tabAlignment: TabAlignment.start,
isScrollable: true,
tabs: <Widget>[
Tab(text: 'TAB 1'),
Tab(text: 'TAB 2'),
],
),
),
body: const TabBarView(
children: <Widget>[
SizedBox.expand(),
SizedBox.expand(),
],
),
floatingActionButton: FloatingActionButton(
onPressed: () {},
child: const Icon(Icons.add),
),
),
),
);
}
}
```
</details>
### Before (`dividerColor: Colors.transparent`, `tabAlignment: TabAlignment.start`)
### After (`dividerColor: Colors.transparent`, `tabAlignment: TabAlignment.start`)
33599006e7