Don't use zero as null pointer constant (-Wzero-as-null-pointer-constant)

Qt-only changes.

src/qt/addresstablemodel.h

@@ -25,7 +25,7 @@ class AddressTableModel : public QAbstractTableModel
     Q_OBJECT
 
 public:
-    explicit AddressTableModel(WalletModel *parent = 0);
+    explicit AddressTableModel(WalletModel *parent = nullptr);
     ~AddressTableModel();
 
     enum ColumnIndex {