Du behöver bara en left outer join
istället för en inre sammanfogning:
SELECT d.date, count(se.id)
FROM (SELECT to_char(date_trunc('day', (current_date - offs)), 'YYYY-MM-DD') AS date
FROM generate_series(0, 365, 1) AS offs
) d LEFT OUTER JOIN
sharer_emailshare se
ON d.date = to_char(date_trunc('day', se.created), 'YYYY-MM-DD'))
GROUP BY d.date;