Send Blank/null parameters to a MII transaction that makes a call to BAPI

At first, believe I investigated in a lot of places, and I have not find nothing that can help me.

Im creating a web page that wants to call the production orders with different search criteria.

Im using BAPI_PRODORD_GET_LIST, to get the orders list. Transaction works fine.

My problem is that not all the times I need to send the same parameters, and sometimes some of them need to be blank or null, depending the criteria.

For instance. Criteria that i want is by plant, workcenter, order type. When I send all the parameters is not problem. But next time i dont want to send the parameter OrderType, because I want all the orders of any type. How can i do that?

Do I have to create several calls in the same transaction? create several transactions ? or make a filter?

Thank you for your help in advance