Merge branch 'dt/http-empty-auth' into maint

http.emptyauth configuration is a way to allow an empty username to
pass when attempting to authenticate using mechanisms like
Kerberos.  We took an unspecified (NULL) username and sent ":"
(i.e. no username, no password) to CURLOPT_USERPWD, but did not do
the same when the username is explicitly set to an empty string.

* dt/http-empty-auth:
  http: http.emptyauth should allow empty (not just NULL) usernames

http.c

@@ -351,7 +351,7 @@ static int http_options(const char *var, const char *value, void *cb)
 
 static void init_curl_http_auth(CURL *result)
 {
-	if (!http_auth.username) {
+	if (!http_auth.username || !*http_auth.username) {
 		if (curl_empty_auth)
 			curl_easy_setopt(result, CURLOPT_USERPWD, ":");
 		return;