Coerce empty string values to `None` for `UV_PYTHON` env var (#7878)
## Summary
Closes #7841. If there are other env vars that would also benefit from
this value parser, please let me know and I can add them to this PR.
## Test Plan
When running the same example from the linked issue, it now works:
```
UV_PYTHON= cargo run -- init x
Compiling ...
Finished `dev` profile [unoptimized + debuginfo] target(s) in 29.06s
Running `target/debug/uv init x`
Initialized project `x` at `/Users/krishnanchandra/Projects/uv/x`
```
