🚀 Feature: Allow hydration/migration to run on an empty directory

[JoshuaKGoldberg]

Bug Report Checklist

• I have tried restarting my IDE and the issue persists.
• I have pulled the latest main branch of the repository.
• I have searched for related issues and found none that matched my issue.

Overview

"Migration" (before #668, "hydration") right now requires a directory to at least have a package.json and be a Git repo (i.e. initialized with git init or git clone). But it wouldn't be much work to be able to run on an empty directory. That'll make testing the hydration/migration script out easier.

Additional Info

Splitting out of the changes from #668.

[github-actions]

🎉 This is included in version v1.28.56 🎉

The release is available on:

• GitHub releases
• npm package (@latest dist-tag)

Cheers! 📦🚀