url: use blank credentials when using proxy w/o username and password

Fixes proxy regression brought in commit ad829b21ae (7.71.0)

Fixed #5911
Closes #5914

lib/url.c

@@ -2379,8 +2379,10 @@ static CURLcode parse_proxy(struct Curl_easy *data,
 static CURLcode parse_proxy_auth(struct Curl_easy *data,
                                  struct connectdata *conn)
 {
-  char *proxyuser = data->set.str[STRING_PROXYUSERNAME];
-  char *proxypasswd = data->set.str[STRING_PROXYPASSWORD];
+  const char *proxyuser = data->set.str[STRING_PROXYUSERNAME] ?
+    data->set.str[STRING_PROXYUSERNAME] : "";
+  const char *proxypasswd = data->set.str[STRING_PROXYPASSWORD] ?
+    data->set.str[STRING_PROXYPASSWORD] : "";
   CURLcode result = CURLE_OK;
 
   if(proxyuser)