Mitra,
Like I said in previous thread, it's possible to have a null value with macro. If you use a macro, you can define a prompt of date in string datatype. In this case, you can accept null value or a date format. In the filter you must transform a string in date type or transform a date in string format.
You can do this when you know that date is validate by user interface. In this case, you that string is in a right date format.
Gilles.