Improve messages for empty solves and installs (#6588)
## Summary
Tries to improve the following:
```
❯ cargo run sync
Compiling uv-cli v0.0.1 (/Users/crmarsh/workspace/uv/crates/uv-cli)
Compiling uv v0.3.3 (/Users/crmarsh/workspace/uv/crates/uv)
Finished `dev` profile [unoptimized + debuginfo] target(s) in 3.81s
Running `/Users/crmarsh/workspace/uv/target/debug/uv sync`
Using Python 3.12.1
Creating virtualenv at: .venv
Resolved in 7ms
Audited environment in 0.05ms
```
In this case we don't actually have any dependencies -- should we just
omit `Resolved in...` and perhaps even the audited line?
