Empty String To Null

example
create view v_GuestOrder
as
Select T1.id_Guest,T2.OrderName
from Guest T1
left join Order T2 T2.id_order = T1.Id_order


select * from v_GuestOrder
--
Id_Guest OrderName
-------- -----
1 spoon
2 phone
3
4 tv

I need something similar to

Select
id_Guest,
case orderName
when '' then Null -- Sql server gives error in thsi case
end as orderName
from v_GuestOrder

So I need to assign NULL to OrderName is query return empty string,
it will be treated by Crystal reports as Null

Please help , thanks