fix: correctly replace app name in file path

[vinzscam]

Summary

Fixed an issue causing file paths to lose the first slash when using RnDiffApp as app name or an empty string after editing the input. A similar fix might need to be applied to appPackage, but I refrained from doing so to avoid potential side effects on any underlying component, as the value can be nullable.

I know there was already #379 open, but we used this fix in our fork and we thought it was good to contribute upstream. Feel free to ignore this PR if another fix is more suitable 😊

What are the steps to reproduce?

1. Type: RnDiffApp in the app name input
2. Press "Show me how to upgrade!"
3. You will see that the path of each file is wrong.

or

1. Type anything in the app name input
2. Clear the app name input
3. Press "Show me how to upgrade!"
4. You will see that the path of each file is wrong.

Checklist

• I tested this thoroughly
• I added the documentation in README.md (if needed)

[pvinis]

could you apply these to make things more typescript-y, and we can merge. thank you for this fix!