rather than change it to 0, it removes it. on integers, it's the same result...
see this simple example
create table #t (a int, b int);
insert #t
select 1, 1 union all
select 1, 1 union all
select 1, 1 union all
select 2, 1 union all
select 2, null union all
select 2, 1
select a, sum(b)
from #t
group by
a
drop table #t