[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.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D11723