Filters - Report Builder

I have created a report showing both first name and last names with the following filters. Both of them are prompted parametres

Last Name Contains ------------------
First Name Contains ------------------

When users enters both then we get the results back. I want the first name to be optional such that if user enters something for the first name then take it into the search criteria else show all the first names which matches the last name criteria.

Example: Last Name = "SMITH", First Name = (user has not entered and NULL check box is checked). I don't get any results since this filter translates to find all the records which has the last name SMITH and First Name IS NULL),

Instead I want this to be select all the records which has the last name SMITH and First Name <> EMPTY. Is there a way I can do this using the fiter magic?

Thanks.