In the `transform` function to `TextEncoderStream`'s internal
`TransformStream`, if `chunk` is the empty string and
`this.#pendingHighSurrogate` is null, then `lastCodeUnit` will be NaN.
As it turns out, this does not cause a bug because the comparison to
check for lone surrogates turns out to be false for NaN, but to rely on
it makes the code brittle.