Merge #15114: Qt: Replace remaining 0 with nullptr

3a0e76fc12 Replace remaining 0 with nullptr in Qt code (Ben Woosley)
9096276e0b 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`.

Tree-SHA512: 206bd668802147ba42bc413c2d7d259cb59aca9ec1da74a6bf2ca3932e60ae492faacbc61bcee0fd6b4b49a4d59d075b7e5404f0526b36c47718f9b0587e7768

src/test/allocator_tests.cpp

@@ -105,13 +105,13 @@ BOOST_AUTO_TEST_CASE(arena_tests)
     // Go entirely wild: free and alloc interleaved,
     // generate targets and sizes using pseudo-randomness.
     for (int x=0; x<2048; ++x)
-        addr.push_back(0);
+        addr.push_back(nullptr);
     uint32_t s = 0x12345678;
     for (int x=0; x<5000; ++x) {
         int idx = s & (addr.size()-1);
         if (s & 0x80000000) {
             b.free(addr[idx]);
-            addr[idx] = 0;
+            addr[idx] = nullptr;
         } else if(!addr[idx]) {
             addr[idx] = b.alloc((s >> 16) & 2047);
         }
@@ -146,7 +146,7 @@ public:
 
             return reinterpret_cast<void*>(0x08000000 + (count<<24)); // Fake address, do not actually use this memory
         }
-        return 0;
+        return nullptr;
     }
     void FreeLocked(void* addr, size_t len) override
     {