[WebDriver BiDi] Fix test for window.prompt opened

[Lightning00Blade]

In the HTML IDL - https://html.spec.whatwg.org/#the-window-object the function is defined as:
prompt(optional DOMString message = "", optional DOMString default = "");
This means that if no value is passed it get empty string.
I also observed that all values passed to the second parameter get toString(ed) (I think this is the algorithm that does it https://webidl.spec.whatwg.org/#js-DOMString),
Verified this behavior for Chrome, Firefox and Safari.

[lutien]

I think we probably then have to update the BiDi spec, because now it assumes that defaultValue might be null: https://w3c.github.io/webdriver-bidi/#event-browsingContext-userPromptOpened

[Lightning00Blade]

@lutien We need the null check in the spec due to other algorithms (alert, confirm, beforeunload),
not having it send from the HTML spec. And it seems default value value for prompt is always present and String.
So we want the value to be omitted for the other types but our expectation should that it is always set for prompt.

[lutien]

@lutien We need the null check in the spec due to other algorithms (alert, confirm, beforeunload), not having it send from the HTML spec. And it seems default value value for prompt is always present and String. So we want the value to be omitted for the other types but our expectation should that it is always set for prompt.

Ah, alright, that sounds reasonable to me. Thanks!