Fix fetch of credentials when cache is seeded with username (#3206)
Fixes the failure to lookup credentials in
https://github.com/astral-sh/uv/issues/3205
The issue is that we seed the cache with the index URL which includes a
username but no password. We did not ensure that a password was present
in the cached credentials before attempting a request with them. Now,
the cache will not return credentials when a username is provided and
the cached credentials have no password — the cached credentials are
useless in that case.
Tested with a Google Artifact Registry and keyring
```
RUST_LOG=uv=trace cargo run -q -- pip install requests --index-url https://oauth2accesstoken@us-central1-python.pkg.dev/<project>/pypi/simple/ --no-cache --keyring-provider subprocess -v
```
