fix: avoid type error (arg must be int, null given)

Author: boite

src/UserProvider.php

@@ -43,7 +43,7 @@ public function loadUserByUsername($username)
         } catch (RuntimeException $e) {
             throw new UsernameNotFoundException(
                 "A User named \"{$username}\" cannot be found in Userbase.",
-                null,
+                0,
                 $e
             );
         }