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

Qt-only changes.

src/qt/receivecoinsdialog.h

@@ -39,7 +39,7 @@ public:
         MINIMUM_COLUMN_WIDTH = 130
     };
 
-    explicit ReceiveCoinsDialog(const PlatformStyle *platformStyle, QWidget *parent = 0);
+    explicit ReceiveCoinsDialog(const PlatformStyle *platformStyle, QWidget *parent = nullptr);
     ~ReceiveCoinsDialog();
 
     void setModel(WalletModel *model);