how to exclude null values in array_agg like in string_agg using postgres?

If I use array_agg to collect names, I get my names separated by commas, but in case there is a null value, that null is also taken as a name in the aggregate. For example :

SELECT g.id,
       array_agg(CASE WHEN g.canonical = 'Y' THEN g.users ELSE NULL END) canonical_users,
       array_agg(CASE WHEN g.canonical = 'N' THEN g.users ELSE NULL END) non_canonical_users
FROM groups g
GROUP BY g.id;

it returns ,Larry,Phil instead of just Larry,Phil (in my 9.1.2, it shows NULL,Larry,Phil).

Instead, if I use string_agg(), it shows me only the names (without empty commas or nulls).

The problem is that I have Postgres 8.4 installed on the server, and string_agg() doesn't work there. Is there any way to make array_agg work similar to string_agg() ?