Provide default for parent in JsonListModel::index()

* Follow how QAbstractItemModel defines index(row, column, parent)
  by defaulting the parent to the empty QModelIndex.

Author: Keith Kyzivat

jsonlistmodel.h

@@ -30,7 +30,7 @@ class JsonListModel : public QAbstractItemModel
     Q_INVOKABLE QJSValue get(const QJSValue&) const;
     Q_INVOKABLE QJSValue asArray(bool deepCopy = false) const;
 
-    QModelIndex index(int row, int column, const QModelIndex &parent) const;
+    QModelIndex index(int row, int column, const QModelIndex &parent = QModelIndex()) const;
     QModelIndex parent(const QModelIndex &child) const;
     int rowCount(const QModelIndex &parent = QModelIndex()) const;
     int columnCount(const QModelIndex &parent= QModelIndex()) const;