Fix wrong redirect following authorization step

#163 fix broke redirect after granting or denying client's access. If there is no `redirect_uri`, `null` is stored in `oauth_access_tokens`, which is correct. However, when user grants or denies access, `OAuth2\Controller\AuthorizeController->handleAuthorizeRequest()` redirected to `?code=...`. The fix in this PR checks the value, and if empty, loads client's registered URI.