Move mempool nullptr Assert out of LoadChainstate

Summary:
Partial backport of core#23280:
https://github.com/bitcoin/bitcoin/pull/23280/commits/8715658983a0a07c56513acd8ded8dfc59c5c169

Depends on D12566.

Test Plan:

ninja all check-all

Reviewers: #bitcoin_abc, PiRK

Reviewed By: #bitcoin_abc, PiRK

Differential Revision: https://reviews.bitcoinabc.org/D12567