clang-tidy, qt: Fix modernize-use-default-member-init in headers

See https://clang.llvm.org/extra/clang-tidy/checks/modernize/use-default-member-init.html

src/qt/receivecoinsdialog.h

@@ -51,7 +51,7 @@ public Q_SLOTS:
 
 private:
     Ui::ReceiveCoinsDialog *ui;
-    WalletModel *model;
+    WalletModel* model{nullptr};
     QMenu *contextMenu;
     QAction* copyLabelAction;
     QAction* copyMessageAction;