url: do not continue HTTP authentication with user=None (issue6425)
I initially thought this is a py3-compat bug of passwordmgr._writedebug(),
but actually returning (None, str) pair is wrong at all. HTTP authentication
would continue with user="None" in that case.
Since registering a password of user=None should also be wrong, this patch
simply adds early return.
(binary:application/octet-stream)