Update a salary by 500

i have a table which consist of salary column having salary 200, 300, 400, null i have to update a salary by 500 such that output will become 700, 800, 900 ,500

update table set salary=isnull(salary,0)+500

i believe when there is no salary you should not update the value to 500 hence the query should be.

update table set salary=salary+500