text: Title: Wrong handling of 0 in expires_at
  Body: If the `expires_at` timestamp in a token is 0 the `is_expired()` function erroneously returns `None` instead of `True`.
  The check should explicitly check for `None`.
  
  https://github.com/lepture/authlib/blob/ee4337cf7c825349dd23870822a3cc7df123097f/authlib/oauth2/rfc6749/wrappers.py#L13-L17