wallet: make chain optional for CWallet::Create
Summary:
This concludes backport of core#20733
https://github.com/bitcoin/bitcoin/pull/20773/commits/489ebb7b34c403a3ce78ff6fb271f8e6ecb47304
Depends on D17644
Test Plan: ninja all check-all
Reviewers: #bitcoin_abc, roqqit
Reviewed By: #bitcoin_abc, roqqit
Differential Revision: https://reviews.bitcoinabc.org/D17645