Mm right, when we first render the output, `imageUrl` is `undefined`, so the output textbox renders with `value={undefined}`, which is an "uncontrolled" component that the DOM is free to change.
In practice, this isn't an issue because the textbox has `isReadOnly`, so the user can't _actually_ change it. But it's still a good idea for consistency and clarity to use an empty string instead of `undefined`, and it removes warning spam from my console!
