QmlDesigner:PropertyEditor: Workaround for empty file model

If there are no files we return a string list containing an empty string.

Task-number: QTCREATORBUG-11327
Change-Id: Idd6680b9b2d9c702c2267796695ef60de0d95fab
Reviewed-by: Robert Loehning <robert.loehning@digia.com>