Not respecting closed stdin

When Python process is launched with a closed stdin descriptor, `sys.stdin` object is initialized as regular instead of being set to `None`.

For demonstration purpose, `ghcr.io/astral-sh/uv:debian` Docker image is used.

```sh
$ uv run --managed-python python -c 'import sys; print(sys.stdin)' <&-
<_io.TextIOWrapper name='' mode='r' encoding='utf-8'>
$ /usr/bin/python3 -c 'import sys; print(sys.stdin)' <&-
None
```

Ideally this behavior should be consistent with reference CPython implementation.

Oddly enough, using `--no-managed-python` with `uv run` also produces `TextIOWrapper` object. As well as running the installed Python directly without invoking `uv`.