How do i pass a NULL value in a SqlDataSource?

How do i pass a NULL value in a SqlDataSource?

My stored procedure has a IF statement where it passes a value if @LastName has a string, but if its empty, @LastName is NULL and SELECT statement executes without the condition.

I want it so that the user enters nothing, it passes NULL, but if the user enters text, the text passes to the @LastName parameter.  

When I enter something in the textbox it works, but if I leave it blank it does not execute the SELECT statement without the conidition.

I tried using the code below....

Any ideas?

Stored Procedure:

set ANSI_NULLS ON
set QUOTED_IDENTIFIER ON
go
ALTER PROCEDURE [dbo].[sp_StoredProcedure]
      @LastName                  varchar(50)      = NULL
AS
BEGIN
      SET NOCOUNT ON;

      DECLARE @sql            nvarchar(4000),
                  @paramlist      nvarchar(4000)      

      SET @sql = 'SELECT  e.FirstName, e.LastName FROM Employees'

      IF @LastName IS NOT NULL
      BEGIN
         SET @sql = @sql + N' AND e.LastName LIKE @xLastName + ''%'''
      END

      SET @paramlist = '@xLastName varchar(50)'

      EXEC sp_executesql @sql, @paramlist, @LastName
END

if(uxLastNameTextbox.Text != "")
      uxEmployeeSqlDataSource.SelectParameters["LastName"].DefaultValue = uxLastNameTextbox.Text;
else
      uxEmployeeSqlDataSource.SelectParameters["LastName"].DefaultValue = null;

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