[backport#17261 3/13] Refactor: Allow LegacyScriptPubKeyMan to be null

Summary:
In CWallet::LoadWallet, use this to detect and empty wallet with no keys

This commit does not change behavior.

https://github.com/bitcoin/bitcoin/pull/17261/commits/eb81fc3ee58d3e88af36d8091b9e4017a8603b3c

Depends on D7838

Partial backport of Core PR17261

Test Plan:

export CC=clang CXX=clang++; cmake -GNinja -DCMAKE_BUILD_TYPE=Debug
ninja all check check-functional

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D7848