This commit fixes an issue where opening zed using `--user-data-dir`
with an empty directory would cause the first run to display a "Failed
to migrate settings" error.

This was caused by the migrator attempting to migrate an empty string,
so if that's the case, we'll simply return `Ok(None)` and avoid
attempting to migrate anything at all.

Relates to #39400

Release Notes:

- N/A

Co-authored-by: Smit Barmase <heysmitbarmase@gmail.com>