Fix issue with AppName stripping incorrectly.

[blakef]

Handles cases where appName wasn't playing nicely with it's default value.

Summary

The appName is meant to be stripped off in various places. When setting the default, the actual value passed around is ''. This change does 2 things:

1. makes regex stricter to only match against prefixes, and
2. passes the default appName down the tree when an empty string is inputted.

Before

After

https://github.com/react-native-community/upgrade-helper/assets/49578/5b467ea7-2415-4879-a40b-4bf4cbeb701d

Test Plan

yarn start

What are the steps to reproduce?

You can see it live.

Checklist

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