Fix paranext-core/extensions vite build failure if there's an empty folder in extensions folder

[tjcouch-sil]

If the extensions/lib folder has an extra folder in it with no contents (or not many contents, particularly a manifest.json file), the Vite build will crash, and it is quite difficult to discover this issue.

This can easily occur if you make a branch where you are making a new extension, then you commit and switch off that branch. Since git doesn't care about directories, that empty extension directory will be left over.

[katherinejensen00]

Check to see if this error still occurs in Webpack

[tjcouch-sil]

This issue occurs on this line and is fixed in #486