Coerce empty string values to None for UV_PYTHON env var

[krishnan-chandra]

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`

[charliermarsh]

It looks like this will error. Do we want it to error, or is it supposed to "unset" the env var? Do you know what other tools do?

[zanieb]

I think we should silently ignore the variable if it's empty, generally. I am curious what other tools do though.

[krishnan-chandra]

Good question. I don't know what other tools do here but I think it feels reasonable to coerce empty strings to None rather than having them error out - I'm happy to make that change here.

[charliermarsh]

We have this pattern for UV_INDEX_URL -- we wrap it in a Maybe type, where it's Maybe::None when empty. It has to be distinct from Option because Clap has special handling for Option.

[krishnan-chandra]

I implemented your suggestion re: Maybe - I think I did this properly but let me know if I didn't.

[charliermarsh]

Thanks, that's great!