test: test resolveObject with an empty path

Add a case to test an URL object that has no path at all
for `url.resolveObject`.

PR-URL: https://github.com/nodejs/node/pull/11811


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yuta Hiroto <hello@about-hiroppy.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>