Adding Null Values for Multi-Valued Parameters - SSRS

When we use multi-valued parameters, we would want to have null values as an option as we may want it to be selected by default. The problem is, there is no null value for multi-valued parameters. One possible solution could be to create a view that has a null value. To do this, we can make use of the UNION ALL command present in SQL.

CREATE VIEW viewDomainWithNull
AS SELECT -1 AS [Domain Id], 'NULL' AS [Domain Name]
UNION ALL
SELECT [Domain Id],[Domain Name] FROM Domain

Taken from
http://www.codeproject.com/KB/reporting-services/SSRSaaka3.aspx