urlapi: preserve empty markers in relative URLs

[Alb3e3]

What

Relative URL resolution currently loses explicit empty query/fragment markers when those markers are needed to compute the splice point.

Example before this patch:

curl_url_set(u, CURLUPART_URL, "http://host/path?#", 0);
curl_url_set(u, CURLUPART_URL, "#new", 0);
curl_url_get(u, CURLUPART_URL, &url, CURLU_GET_EMPTY);

Expected:

http://host/path?#new

Actual before this patch:

http://host/path#new

The empty query marker is significant when the caller asks for empty components with CURLU_GET_EMPTY.

This patch fixes the relative-resolution path by:

• preserving empty query/fragment separators when serializing the existing base URL for relative resolution
• using query_present and fragment_present when choosing the cutoff point, instead of only checking for non-empty query/fragment strings
• adding focused unit1675 coverage for fragment-only, query-only, and path-relative references across explicit empty query/fragment markers

Tests

cmake --build _build --target units -j2 && ./_build/tests/unit/units unit1675

perl ../../tests/runtests.pl -c ../src/curl -a -n 1675

I also checked the public URL API behavior with a small local probe for the examples above.

Disclosure: AI assistance was used while inspecting and drafting this patch; the behavior and focused tests listed above were verified locally.

[bagder]

This looks like a perfectly fine change, thanks! I would however prefer the test code to be added to test 1560 instead (lib1560.c) since it can be a libtest, not a unit test, and 1560 is already doing most of the URL API testing.

[Alb3e3]

Moved the URL API regression coverage into lib1560: the existing #, ?, and ?# cases now assert explicit markers via CURLU_GET_EMPTY, and all five relative-resolution cases from unit1675 are covered in set_url_list. Removed those public-API cases from unit1675. Focused lib1560 and unit1675 runs pass.

[bagder]

Thanks!