text: Title: Requesting empty scope removes scope from response
  Body: When this [request is made](https://httpie.org/doc#forms):
  
      http -a client:secret -f :/auth/token grant_type=client_credentials scope=
  
  I get response, without `scope` even if it was given in request.
  
  Code responsible for this is here:
  
  https://github.com/lepture/authlib/blob/master/authlib/oauth2/rfc6750/wrappers.py#L98-L99
  
  Is this a bug? I would expect `scope` present in response since it was given in request, even if given scope was empty string.