[backport] Qt: Replace remaining 0 with nullptr #15114

Summary:
3a0e76fc12b91b2846d756981e15f09b767a9c37 Replace remaining 0 with nullptr in Qt code (Ben Woosley)
9096276e0b2d5b7e19af9a5f3c144ef108ee55e0 Don't use zero as null pointer constant (-Wzero-as-null-pointer-constant) (practicalswift)

Pull request description:

This corrects all violations of -Wzero-as-null-pointer-constant identified in the Qt codebase.

These changes are extracted from #15112 as suggested by @MarcoFalke to ease review. This is in service of enabling -Wzero-as-null-pointer-constant, which should eliminate this as a concern going forward.

Note there are 2 non-Qt changes: src/test/allocator_tests.cpp and src/wallet/db.cpp.

This is a backport from Core PR15114 (https://github.com/bitcoin/bitcoin/pull/15114)

Test Plan:

ninja check
ninja check-functional

Reviewers: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Reviewed By: O1 Bitcoin ABC, #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5341