`func.unnest` loses type information

Given a column expression with type `ARRAY<T>`, calling `func.unnest(expr)` should result in a column of type `T`, nit it results in `NullType` instead.

```
>>> am.c.objects.type
ARRAY(JSON())
>>> select(func.unnest(am.c.objects).alias('another_mock_objects').column).subquery().c.another_mock_objects.type
NullType()
```
