Merge #15114: Qt: Replace remaining 0 with nullptr
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`.