`apply()` nullability bug

[connortsui20]

What happened?

apply() has a bug with null propagation from struct validity to non-nullable child fields.

This test should pass, but it panics now.

vortex-array/src/scalar_fn/fns/get_item.rs

    fn get_nullable_field() {
        let st = StructArray::try_new(
            FieldNames::from(["a"]),
            vec![buffer![1i32].into_array()],
            1,
            Validity::AllInvalid,
        )
        .unwrap()
        .into_array();

        let get_item_expr = get_item("a", root());
        let item = st.apply(&get_item_expr).unwrap();
        // The dtype should be nullable since it inherits struct validity
        assert_eq!(
            item.dtype(),
            &DType::Primitive(PType::I32, Nullability::Nullable)
        );
    }

Steps to reproduce

See tests above

Environment

All environments

Additional context

#7844