LOV Default

How does a default Oracle report parameter work? 

Let's say you have a table that has a status with values blank, A, B, and C and the parameter is the status.

The query would be select status from table where status = <parm>.  If the parameter isn't entered, statuses blank, A, B. and C should be displayed.

How do I ensure that I get this result set, when the parameter isn't entered:

select status from table where status in (blank,A,B,C)

and not this result set:

select status from what where status IS NULL
 

Thanks.