QmlDesigner: Use null instead of 0 for Item

Change-Id: Ifd6f06642308a981f7c50604ac1f1db56eabc261
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: <github-actions-qt-creator@cristianadam.eu>
Reviewed-by: Miikka Heikkinen <miikka.heikkinen@qt.io>

src/plugins/qmldesigner/components/connectioneditor/dynamicpropertiesmodel.cpp

@@ -102,7 +102,7 @@ QVariant convertVariantForTypeName(const QVariant &variant, const QmlDesigner::T
     } else if (typeName == "alias") {
         returnValue = "null";
     } else if (typeName == "Item") {
-        returnValue = 0;
+        returnValue = "null";
     }
 
     return returnValue;