Select CASE when null ?

Hi list :) How are you today?

Being fast: I have the following table with the following data in it:

users:
mid --- id_group --- username
1 ----- 2 --- test
2 ----- 2 --- blabla
3 ----- 4 --- etcetc

and the following select:

SELECT
CASE mid WHEN NULL THEN CAST(0 AS integer)
ELSE mid
END AS mid,
CASE id_group WHEN NULL THEN CAST(0 AS integer)
ELSE id_group
END AS id_group
FROM users
WHERE username = 'test';

This query returns:
mid --- id_group
1 --- 2

Now, what I want is when the user isn't found, (aka WHERE username isn't
found) it should return me this:

mid --- id_group
0 --- 0

But it returns 0 rows...

I've tried so far with
CASE mid WHEN NOT FOUND -> ERROR: column "found" does not exist
CASE mid WHEN NOT EXISTS -> ERROR: column "exists" does not exist
and other variations, however, I haven't be able to accomplish this, the
idea is that it should always return at least 1 row...

I know it must be a some kind of small stupidity, but after 2 hours googling
and trying, i seriously thought of asking it to the list...

Thanks in advance ;)

Greetings, Camilo Sperberg

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