gh-76960: Fix urljoin() and urldefrag() for URIs with empty components (GH-123273)

* urljoin() with relative reference "?" sets empty query and removes fragment.
* Preserve empty components (authority, params, query, fragment) in urljoin().
* Preserve empty components (authority, params, query) in urldefrag().

Also refactor the code and get rid of double _coerce_args() and
_coerce_result() calls in urljoin(), urldefrag(), urlparse() and
urlunparse().