Finally found the answer:
[dbo].[USP_GetProjectPhase] @PurchaseOrder INT
AS
SELECT -1 AS 'ProjectPhaseID'
,'No Filter' AS 'Phase'
UNION
SELECT pp.ProjectPhaseID
,pp.Phase
FROM ProjectPhase pp
WHERE @PurchaseOrder = pp.PurchaseOrderId
In my query I changed the WHERE clause to:
WHERE (reg.ProjectPhaseId IN (SELECT Value FROM fnLocal_CmnParseList(@Phase,',')) OR @Phase="-1")
1
solved NULL values in multivalue parameter