refactor: Avoid magic value of all-zeros in assumeutxo base_blockhash

Summary:
Just use std::optional

This concludes backport of core#21584
https://github.com/bitcoin/bitcoin/pull/21584/commits/fa340b87944764ea4e8e04038fe7471fd452bc23

Depends on D11334 and D11241

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11331