In previous versions of authlib using `client_secret_basic` without a
`client_secret` would result in an invalid auth header. Since authlib
1.3 it throws an exception.
The configuration may be accepted in by very lax servers, so we don't
want to deny it outright. Instead, let's default the
`client_auth_method` to `none`, which does the right thing. If the
config specifies `client_auth_method` and no `client_secret` then that
is going to be bogus and we should reject it
