[avalanche] Send a avahello message when no proof is supplied
Summary: This will let our peer know that we do support avalanche despite not having any proof attached. This is achieved by sending a dummy delegation in the message.
ninja all check-all
Reviewers: #bitcoin_abc, sdulfari
Reviewed By: #bitcoin_abc, sdulfari
Differential Revision: https://reviews.bitcoinabc.org/D11723