Optimization

Hello All,
What is the best way to optimize this code or rewrite it using ISNULL ?

CREATE PROCEDURE get_employees (@dept char(8), @class char(5))
AS
IF (@dept IS NULL AND @class IS NOT NULL)
SELECT * FROM employee WHERE employee.dept IS NULL
AND employee.class=@class
ELSE IF (@dept IS NULL AND @class IS NULL)
SELECT * FROM employee WHERE employee.dept IS NULL
AND employee.class IS NULL
ELSE IF (@dept IS NOT NULL AND @class IS NULL)
SELECT * FROM employee WHERE employee.dept=@dept
AND employee.class IS NULL
ELSE
SELECT * FROM employee WHERE employee.dept=@dept
AND employee.class=@class

Thanks in advance.
-Praveena