Vector runtime: `sum` aggregate with filter produces typed null instead of untyped null

[mattnibs]

Summary

When using sum with a filter clause inside aggregate, the vector runtime produces null::int64 (typed null) instead of null (untyped null) for groups that are entirely filtered out.

Reproduction

echo '{k:"foo",v:1} {k:"bar",v:2} {k:"foo",v:3} {k:"baz",v:4}' | super -s -c 'sum(v) filter (k=="bar") by key:=k | sort' -

Expected:

{key:"bar",sum:2}
{key:"baz",sum:null}
{key:"foo",sum:null}

Actual (vector):

{key:"bar",sum:2}
{key:"baz",sum:null::int64}
{key:"foo",sum:null::int64}

Test reference

book/src/super-sql/operators/aggregate.md:194

Discovered via make test-heavy which runs documentation examples through the vector runtime.