case statement in sql server

I am trying to retrieve students who are ferpa restricted or not.

(1) When I enter 'y' it should give me a list of ferpa restricted students.

(2) When I enter 'n' it should give me a list of ferpa restricted students.

(3) When It's blank. It should give me both types of student.

I could fetch student with option 'y' and 'n' but what about option 3.

I am using following query

SELECT DISTINCT name_master.id_num,
name_master.first_name,
name_master.last_name,
name_master.is_ferpa_restricted,
stud_term_sum_div.yr_cde,
stud_term_sum_div.trm_cde
FROM name_master,
stud_term_sum_div
WHERE ( name_master.id_num = stud_term_sum_div.id_num ) and
( ( stud_term_sum_div.yr_cde = '2016') AND
( stud_term_sum_div.trm_cde = '20' ) ) and
NAME_MASTER.IS_FERPA_RESTRICTED in (case IS_FERPA_RESTRICTED when 'y' then 'y'
...