Quartiles result zero values

I have a task to find quartiles value from this data:

table's name : student

Here is the sql:

select p.major AS ProgramStudi,
       count(usia) as N, AVG(usia) as Mean,
       MIN(usia) as Minimum, MAX(usia) as Maximum,
       STDDEV(usia) as stddev,
       max(case when floor(0.25*pp.cnt) = rn then usia end) as quartile_1,
       max(case when floor(0.50*pp.cnt) = rn then usia end) as quartile_2,
       max(case when floor(0.75*pp.cnt) = rn then usia end) as quartile_3
from (select p.*,
             (@rn := if(@m = p.major, @rn + 1,
                        if(@m := p.major, 1, 1)
                       )
             ) as rn
      from student p cross join
           (select @m := '', @rn := 0) params
      order by p.major, p.usia
     ) p join
     (select p.major, count(*) as cnt
      from student p
      group by p.major
     ) pp
     on p.major = pp.major
group by p.major;

and the result goes like this:

Why can there be zero values on quartiles? What may I do to fix the zero values? Thanks in advance