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