Use less nullptr for empty flags

Change-Id: Ic4eafdc8f204a432a752a97593380609a408a7de
Reviewed-by: Eike Ziller <eike.ziller@qt.io>

src/plugins/debugger/watchhandler.cpp

@@ -1178,7 +1178,7 @@ bool WatchModel::setData(const QModelIndex &idx, const QVariant &value, int role
 Qt::ItemFlags WatchModel::flags(const QModelIndex &idx) const
 {
     if (!idx.isValid())
-        return nullptr;
+        return {};
 
     const WatchItem *item = nonRootItemForIndex(idx);
     if (!item)