[avalanche] Don't turn on service bit if we don't have a proof

Summary:
A node with no proof cannot participate actively to the avalanche
network, so it might as well not advertise it with the service bit.

Depends on D9440.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D9422