[Language Detector] Add handling for empty expectedInputLanguages

Updates implementation and adds test to check that
`detector.expectedInputLanguages` is `null` when an empty array is
passed in.

Spec:
https://webmachinelearning.github.io/translation-api/#create-the-language-detector-object

Bug: 414814251
Change-Id: Ibb9086c94ea02227eba8d1ead62747fcaa2a2c43
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/6501483
Commit-Queue: Christine Hollingsworth <christinesm@chromium.org>
Reviewed-by: Nathan Memmott <memmott@chromium.org>
Cr-Commit-Position: refs/heads/main@{#1454662}

Author: christine-hollingsworth

ai/language_detection/detector.https.window.js

@@ -184,6 +184,11 @@ promise_test(async t => {
   });
 }, 'Aborting LanguageDetector.measureInputUsage().');
 
+promise_test(async () => {
+  const detector = await createLanguageDetector({expectedInputLanguages: []});
+  assert_equals(detector.expectedInputLanguages, null);
+}, 'Creating LanguageDetector with empty expectedInputLanguages array');
+
 promise_test(async () => {
   const detector = await createLanguageDetector();
   assert_equals(detector.expectedInputLanguages, null);