default null syntax

Motivation: when `?.` is means we can call `select` on a `nullable` value, but it still throws `Field does not exists` exception.

What about using a `obj.b?` or `obj['b'?] syntax to express: if `b` does not exists in `obj` , then return `null` instead of an error.

- obj.b?
- obj["b"?]

or, we can just use `std.get` method