refactor: Introduce GetFirstStoredBlock helper function

Summary:
This is a partial backport of core#21726
https://github.com/bitcoin/bitcoin/pull/21726/commits/231fc7b035481f748159968353c1cab81354e843

Depends on D13096

Test Plan: ninja all check-all

Reviewers: #bitcoin_abc, sdulfari

Reviewed By: #bitcoin_abc, sdulfari

Differential Revision: https://reviews.bitcoinabc.org/D13097