Stored Procedure null parameter within where clause
SELECT *
FROM Table1
WHERE Table1.URL LIKE '%' + @Parameter1 + '%' AND Table1.ID = @Parameter2
AND 
(
    @Parameter3 is null 
    or Table1.ID2 = @Parameter3
);


Take a look at the above example.  If you change your AND clause to a nested OR clause specifying your initial expression as well as @Parameter3 is null.  That will then demand that the nested expression is true if @Parameter3 is NULL.