Pushdown null check filter below agg

[kaikalur]

select * from (SELECT arbitrary(x) x, y from t group by y) where x is not null

we apply the not null after the aggregation. But if this is the only aggregation, we can actually apply the not null much sooner right after scan like:

select * from (SELECT arbitrary(x) x, y from t where x is not null group by y)

bonus: do it if there are multiple "return null on null" aggs on the same field!