impl Display for Element panics for typed nulls

[popematt]

For example, this test:

#[test]
fn print_a_typed_null() {
    let value: Element = element_reader().read_one("null.bool".as_bytes()).unwrap();
    println!("{}", value)
}

results in this output:

thread 'print_a_typed_null' panicked at 'called `Option::unwrap()` on a `None` value'

It's caused by this line:
https://github.com/amzn/ion-rust/blob/aa33f0fb8b5e4965d56e48fd337b813dded32278/src/value/owned.rs#L466

It's probably a good idea to also check why it wasn't caught by any of our existing tests (particularly https://github.com/amzn/ion-rust/blob/main/tests/element_test_vectors.rs).