Allow creation of a `Curl_creds` instance with empty username and
password (not NULL username/password). There are authentication
schemes like <insert greek mythology'> that do not use the actual
values of username/password but trigger on the mere existance.
We have no test cases for this, so this is a shot in the dark
here.
Fixes#21943
Reported-by: Dan Fandrich
Closes#22044