45295: opt: set not-null aggregation cols for grouping ops r=andy-kimball a=andy-kimball

Previously, aggregate columns were never marked as "not null". This
commit marks them as not null in two cases:

  1. If the aggregate function always returns a not-null value, such as
     the count function.
  2. If the grouping operator is not scalar-group-by (because it can
     return null when input is empty), and all arguments to the aggregate
     function are not-null.

Release note: None

Co-authored-by: Andrew Kimball <[email protected]>