[avalanche] Ensure nullptr safety on proccesor's public interface

Summary: This needs fixing because I managed to trigger a segfault while writing a separate patch.

Test Plan:

ninja check-avalanche

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11468