Fixes git graph showing an infinite loading state for repositories with
no commits. The bug happened because an empty `git log --all` result
completed successfully with zero commits, but the UI treated a loaded
graph with `0` commits as still loading instead of showing the empty
state.

Self-Review Checklist:

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

Closes #53399


Release Notes:

- Fixed git graph loading forever for empty repositories.