Don't send emtpy Bearer tokens
Don't send emtpy Bearer tokens

As per spec (https://tools.ietf.org/html/rfc6750#section-2.1) the syntax
for Bearer authentication is as follows:

  b64token    = 1*( ALPHA / DIGIT /
                    "-" / "." / "_" / "~" / "+" / "/" ) *"="
  credentials = "Bearer" 1*SP b64token

"1*" indicating "at least one repetition".

By definition, an OAuth2 "token" request cannot be made using Bearer
authentication. The sent `Authorization` header is therefore invalid and
can be rejected as a HTTP 400 (Bad Request) error.

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