Since NOT NULL is so common it would be great to have tables start to enforce that.
One way to enforce not null constraint is to have the optimizer add enforce_not_null UDF which throws if it sees a NULL, otherwise it's pass through.
So if 'b' has not null constraint,
Insert into T select a,b,c... would become
Insert into T select a, enforce_not_null(b), c.....
This would work for any table type.