Make ion_type() correctly reflect typed nulls

[zslayton]

          Could you put this functionality in a `self.read_null()` method instead of having it happen in `ion_type()`?

There are three other raw value types besides LazyRawBinaryValue_1_1:

• LazyRawBinaryValue_1_0
• LazyRawTextValue_1_1
• LazyRawTextValue_1_0

Each of them has an ion_type() method and I'm pretty sure at least some of them return IonType::Null if the value is a null value, even if it's a typed null. The behavior you've implemented is probably more correct, but if we make this change here we'd need to make it in the other three places too.

Originally posted by @zslayton in #826 (comment)