Leverage Empty Base Optimization for the radix tree Adapter

Summary:
As per title, see https://en.cppreference.com/w/cpp/language/ebo.
Suggested by @deadalnix in D11571.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Subscribers: deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D11574