QmlDesigner.propertyEditor: show emtpy instead of empty string

share/qtcreator/qmldesigner/propertyeditor/Qt/Switches.qml

@@ -20,7 +20,7 @@ QFrame {
             id: standardMode;
             toolTip: "special properties";
             //iconFromFile: "images/rect-icon.png";
-            text: backendValues === undefined ? "empty" : backendValues.className.value
+            text: backendValues === undefined || backendValues.className === undefined || backendValues.className == "empty" ? "empty" : backendValues.className.value
             onClicked: {
                 extendedMode.checked = false;
                 layoutMode.checked = false;