Fix ``uv pip install -r /dev/stdin`` (#16855)
## Summary
Fix ``uv pip install -r /dev/stdin`` which was broken in uv 0.9.12 by
https://github.com/astral-sh/uv/pull/16805 .
Example of the issue:
```
$ echo "flask" | uv pip install -r /dev/stdin
warning: Requirements file `/dev/stdin` does not contain any dependencies
Audited in 8ms
```
Note that "upstream" ``pip install`` does support `-r /dev/stdin` and
doesn't support `-r -` .
## Test Plan
2 new tests added.
