According to the OAuth 2.0 Section 4.1.3, the client_id is 'required' for Access Token Requests when alternate forms of authentication are not provided. Nuxeo should also check the Authorization header for the client and secret information.
Reference:
https://tools.ietf.org/html/rfc6749#section-4.1.3
Alternate authentication:
https://tools.ietf.org/html/rfc6749#section-3.2.1