If you have the table with all the months, select from that as the primary table, and do a left join. like so:
select month, ifnull(count(t.id),0) transactions
from my_month_table m
left join 'tbl_test' t on m.month_key=t.month_key
where id=" + id + " and Month is not NULL and Year = " + Convert.ToInt32(currentYear) + "