Previously, passing an empty range to `range_to_buffer_range` would
return no ranges, which would trigger a fallback case in the git diff
that caused the entire multibuffer to be invalidated every frame.

This PR fixes `range_to_buffer_range`, so that it returns a sensible
range if the input range is empty, preventing the fallback behaviour and
edit expansion.

Self-Review Checklist:

- [ ] I've reviewed my own diff for quality, security, and reliability
- [ ] Unsafe blocks (if any) have justifying comments
- [ ] The content is consistent with the [UI/UX
checklist](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
- [ ] Tests cover the new/changed behavior
- [ ] Performance impact has been considered and is acceptable

Closes #ISSUE

Release Notes:

- N/A or Added/Fixed/Improved ...

---------

Co-authored-by: Cole Miller <cole@zed.dev>
Co-authored-by: Anthony Eid <hello@anthonyeid.me>