SQL statement to get $0 if the customer has no billing from any specific year

Question

I am trying to find the average billing amount per year from 2019 to 2021 for every customer, and I want to return $0 if the customer has no billing from any specific year. I tried doing a left join but unfortunately it does not give the result I need. In the table below, how do I return $0 for customer_id 1 since the year for 2021 does not exist. Thanks.

billing table:

customer_id	billing_id	created_date	billing_amount
1	id_11	2019-06-21	100
1	id_12	2020-05-11	126
1	id_13	2019-12-28	86
2	id_21	2019-12-28	28
2	id_22	2020-12-28	56
2	id_23	2021-12-28	26

Here is my incorrect query:

Select a.customer_id,
       extract(year from a.created_date),
       avg(a.billing_amount)
from billing as a
left join billing as b
    on a.customer_id = b.customer_id 
where a.created_date between '2019-01-01' and '2021-12-31'
group by 1, 2