Re: [w3c/browser-payment-api] Take paymentRequestID in constructor (#413)

The `optional` keyword does not mean nullability, it means omitability. Suppose your have this IDL:

    void func(optional DOMString stuff);

Then your JavaScript code `func(null)` will result in `stuff === "null"`. That's a non-null string of 4 characters whose value is`"null"`, not an actual `null` string. This IDL allows calling `func()`.

Suppose your have this IDL instead:

    void func(DOMString? stuff);

Then your JavaScript code `func(null)` will result in `stuff === null`. That's the actual `null` string. This IDL does not allow calling `func()`.

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