Pass null Chain interface pointer to CWallet. This is needed to drop
libbitcoin_server dependency and avoid linking node code.
https://github.com/bitcoin/bitcoin/pull/15639/commits/fbc6bb8e8310ddf12e675d698cda3bdae4f361b8
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Partial backport of Core [[https://github.com/bitcoin/bitcoin/pull/15639 | PR15639]]