url: prevent pathname setter from erasing path of path-only URLs

This change prevents the pathname setter from erasing the path of
path-only URLs as that would make them cannot-be-a-base URLs.

The changes in all files except `src/node_url.cc` have been done by
running:

```console
git node wpt url
```

Fixes: https://github.com/nodejs/node/issues/39059


Signed-off-by: Darshan Sen <darshan.sen@postman.com>

PR-URL: https://github.com/nodejs/node/pull/39060


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Khaidi Chu <i@2333.moe>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Tiancheng "Timothy" Gu <timothygu99@gmail.com>