Mapping Picklist field in Search Region to NULL value in a table

I have a requirement where in there is a field "Operating Unit" which has a pick list assigned to it.

This field is in a Search Region.

Currently if don't enter any value for this field and do a search , it returns all the records from the table, irrespective of the operating unit.

The requirement is to:

If a user does a search without any value in this field, then it should return only those records from the table, where in the "OPERATING_UNIT" column is NULL.

Can someone please suggest how to achieve this.

Thanks

Sunil