Function Module with optional parameter

Hi Gurus,
I have a requirement where i have to fetch the records from MARC table based on matnr ( importing parameter - Optional ) and werks as table parameter .
When i am writing the select query on MARC , if the material is blank and plant is having data , still the select query is failing.
I want to get even if the material is blank , the select query should get all the materials under that plant.
Kindly help in this regard.
Thanks in Advance.