refactor(ext/web): Don't rely on NaN comparisons in TextEncoderStream (#13151)

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.

ext/web/08_text_encoding.js

@@ -292,6 +292,9 @@
         transform: (chunk, controller) => {
           try {
             chunk = webidl.converters.DOMString(chunk);
+            if (chunk === "") {
+              return PromiseResolve();
+            }
             if (this.#pendingHighSurrogate !== null) {
               chunk = this.#pendingHighSurrogate + chunk;
             }