validation: Don't assume m_chain_tx_count in GuessVerificationProgress

Summary:
In the context of an a descriptor import during assumeutxo background sync, the progress can not be estimated due to m_chain_tx_count being set to 0.

This is a partial backport of core#30909
https://github.com/bitcoin/bitcoin/pull/30909/commits/27f99b6d63b7ca2d4fcb9db3e88ed66c024c59d5

Depends on D17925

Test Plan:
tested via the wallet_assumeutxo.py test on my dev branch ("Test loading descriptors during background sync")

This only affects logging in production builds, no change in behavior

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D17926