Fix vertical Stepper draws connector on the last step (#145703)
fixes [Vertical stepper shows line after last step](https://github.com/flutter/flutter/issues/144376)
### 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: Scaffold(
body: Center(
child: Stepper(
currentStep: 1,
steps: const <Step>[
Step(
title: Text("Step 1"),
content: Text("Content 1"),
),
Step(
title: Text("Step 2"),
content: Text("Content 2"),
),
],
),
),
),
);
}
}
```
</details>
### Before
### After
5e19b33534