http: http.emptyauth should allow empty (not just NULL) usernames

http: http.emptyauth should allow empty (not just NULL) usernames

When using Kerberos authentication with newer versions of libcurl,
CURLOPT_USERPWD must be set to a value, even if it is an empty value.
The value is never sent to the server.  Previous versions of libcurl
did not require this variable to be set.  One way that some users
express the empty username/password is http://:@gitserver.example.com,
which http.emptyauth was designed to support.  Another, equivalent,
URL is http://@gitserver.example.com.  The latter leads to a username
of zero-length, rather than a NULL username, but CURLOPT_USERPWD still
needs to be set (if http.emptyauth is set).  Do so.

Signed-off-by: David Turner <dturner@twosigma.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>