Repeat all base table rows for EACH and don't aggregate the NULL from the join table
You need to create all customer_id/feedback_id combinations first and then left join to it.

Assuming there's a customer table with unique ids:

select x.customer_id, x.feedback_id, x.feedback_desc, cf.given_answers
from
 (
   select c.customer_id, f.feedback_id, f.feedback_desc
   from feedback as f
   cross join customer as c -- or (select distinct customer_id 
                            --     from customer_feedback) as c 
 ) as x
left join customer_feedback as cf
  on x.customer_id = cf.customer_id
 and x.feedback_id = cf.feedback_id ;


or, without a derived table:

select c.customer_id, f.feedback_id, f.feedback_desc, cf.given_answers
from feedback as f
  cross join customer as c -- or (select distinct customer_id 
                           --     from customer_feedback) as c 
  left join customer_feedback as cf
    on  c.customer_id = cf.customer_id
    and f.feedback_id = cf.feedback_id ;