Stored Procedure

I Have a Stored procedure like below

USE [AfmsBizTalkAggregationDb]
GO

/****** Object:  StoredProcedure [dbo].[InsertUnitItem]    Script Date: 06/25/2014 22:11:23 ******/
SET ANSI_NULLS ON
GO

SET QUOTED_IDENTIFIER ON
GO

CREATE PROCEDURE [dbo].[InsertUnitItem]
@Item XML, @ItemType NVARCHAR (255), @CorrelationValue NVARCHAR (25)--='empty'
--Defect#92507:Removed the default parameter passed to filed @CorrelationValue 
--G.Sunitha,23-May-2014
, @DoInsert BIT
AS
BEGIN

-- find typeid
declare @TypeId smallint
select @TypeId = [Id] from [ItemTypes] where Lower(@ItemType)=Lower([Name])

if(@DoInsert = 1) -- Added by HDC team to implement defect# 22502
insert into [dbo].[ItemsByType] (Item, TypeId) values (@Item, @TypeId)

Update ProcessingHistory
Set Flag = 1
where ProcessIdentifier=@CorrelationValue

END




GO

To execute this stored procedure  i have to give it like

exec [dbo].[InsertUnitItem] 'xml','operationcards','98f'

but in my scenario correlation value can be null, if i give like this it should be able to execute

exec [dbo].[InsertUnitItem] 'xml','operationcards'

how can i do this?