test: test resolveObject with an empty path

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

This test increases the coverage of lib/url.js:

• https://coverage.nodejs.org/coverage-055482c21e2df944/root/url.js.html

The following lines will be checked:

• https://github.com/nodejs/node/blob/055482c21e2df94446d9796949d1bb810e8331bf/lib/url.js#L846-L849
• https://github.com/nodejs/node/blob/055482c21e2df94446d9796949d1bb810e8331bf/lib/url.js#L853-L858

This PR was part of https://github.com/nodejs/node/pull/11395 that I created, and I separated a non-breaking change commit from #11395 to this PR. I will update #11395 later.

Checklist

• make -j4 test
• tests is included
• commit message follows commit guidelines

Affected core subsystem(s)

test