Set password segment of user info if not null

Previously, a lazy `if ($password)` check meant that a value of `0`
would be ignored; it now checks more specifically for `if (null !==
$password)`.

Author: weierophinney

src/Uri.php

@@ -291,7 +291,7 @@ public function withUserInfo($user, $password = null)
         }
 
         $info = $this->filterUserInfoPart($user);
-        if ($password) {
+        if (null !== $password) {
             $info .= ':' . $this->filterUserInfoPart($password);
         }