How To Use CASE When Set Query Empty

I have a query that will either return one record or zero records. When it returns zero records I want to replace my attributes with a sentinel, like 'N/A'. I tried the CASE statement but couldn't get anything to work

Sample (does not work):
select
(case when exists (select product from tbl_product where productid = '123') then product else 'N/A' end) product
from tbl_product
where productid= '123';
If one record exists it should produce: 'My Widget' (or whatever)
If zero records exist it should produce: 'N/A'