MUST send for the SETTINGS frame, but there is no specific need to include any values in it.  As we've agreed several times over, we don't want to have the HTTP/2.0 connection start sequence be different based on how we got there.

And sadly, empty isn't allowed, probably because it messes up lexers (in p7 at least).  The definition would have to explicitly allow it.
token68        = 1*( ALPHA / DIGIT / "-" / "." / "_" / "~" / "+" / "/" ) *"="