Do not show empty version specifier in `uv tool list` (#9605)
## Summary
Do not show empty version specifier in `uv tool list
--show-version-specifier`.
Before:
```console
$ uv tool list --show-version-specifiers
ipython v8.29.0 [required: ]
- ipython
- ipython3
maturin v1.7.4 [required: ]
- maturin
pre-commit v4.0.1 [required: ]
- pre-commit
rooster-blue v0.0.8 [required: ]
- rooster
ruff v0.8.0 [required: ]
- ruff
yt-dlp v2024.11.18 [required: ]
- yt-dlp
```
After:
```console
$ cargo run -- tool list --show-version-specifiers
ipython v8.29.0
- ipython
- ipython3
maturin v1.7.4
- maturin
pre-commit v4.0.1
- pre-commit
rooster-blue v0.0.8
- rooster
ruff v0.8.0
- ruff
yt-dlp v2024.11.18
- yt-dlp
```
