to_bytes_until_nul() returns None if no null byte

[nallux-dozryl]

Not sure if it's intended behavior, but it's causing issues in http-app

Reproduce:

use crown::{Bytes};
use crown::{AtomExt};
use sword::noun::{Atom};
use sword::mem::NockStack;

fn main() {
    let atom = Atom::from_bytes(&mut NockStack::new(1 << 27, 0), &Bytes::from("1234567"));
    println!("{:?}", atom.to_bytes_until_nul()); // Some([49, 50, 51, 52, 53, 54, 55])
    let atom = Atom::from_bytes(&mut NockStack::new(1 << 27, 0), &Bytes::from("12345678"));
    println!("{:?}", atom.to_bytes_until_nul()); // None
}