urlapi: add CURLU_GET_EMPTY for empty queries and fragments

urlapi: add CURLU_GET_EMPTY for empty queries and fragments

By default the API inhibits empty queries and fragments extracted.
Unless this new flag is set.

This also makes the behavior more consistent: without it set, zero
length queries and fragments are considered not present in the URL. With
the flag set, they are returned as a zero length strings if they were in
fact present in the URL.

This applies when extracting the individual query and fragment
components and for the full URL.

Closes #13396