feat(optimizer)!: bq annotate type for NULL

[geooo109]

This PR adds type annotation for bq NULL

SELECT typeof(NULL);
> INT64

[georgesittas]

How does this affect coercion? e.g.

select whatever_expr union all select null

I think the type should always coerce to whatever_expr here.

[geooo109]

If you wrap it in a CTE, it works fine.

The UNION ALL expression isn't annotated as is (the expression has unknown type).

We handle the coercion of Set Operations in the annotate_scope, when we have sources (it was added mostly for CTES).

In queries like this select whatever_expr union all select null, we don't apply any coercion between the LHS/RHS. There are cases that make this tricky:

D with tbl as (select 1::INT as a, a+1 as b union all select 1::float as a, NULL as b) select typeof(a), typeof(b) from tbl;
┌───────────┬───────────┐
│ typeof(a) │ typeof(b) │
│  varchar  │  varchar  │
├───────────┼───────────┤
│ FLOAT     │ INTEGER   │
│ FLOAT     │ INTEGER   │
└───────────┴───────────┘