Resolve install() with false for an empty langs sequence.

The prose already states the promise resolves to false if options.langs is
empty, but no step implemented it: an empty sequence vacuously passes the
unsupported-language check and then trivially succeeds with no downloads,
resolving to true. Add an explicit step, matching how the availability
algorithm treats an empty sequence as unavailable.

Author: padenot

index.bs

@@ -422,7 +422,8 @@ See <a href="https://lists.w3.org/Archives/Public/public-speech-api/2012Sep/0072
     1. Let <var>promise</var> be <a>a new promise</a>.
     1. If the [=current settings object=]'s [=relevant global object=]'s [=associated Document=] is NOT [=fully active=], reject <var>promise</var> with an {{InvalidStateError}}, abort these steps, returning <var>promise</var>.
     1. If any <var>lang</var> in {{SpeechRecognitionOptions/langs}} of <var>options</var> is not a valid [[!BCP47]] language tag, reject <var>promise</var> with a {{SyntaxError}}, abort these steps, returning <var>promise</var>.
-    1. If the on-device speech recognition language pack for any <var>lang</var> in {{SpeechRecognitionOptions/langs}} of <var>options</var> is unsupported, resolve <var>promise</var> with false, abort these steps, returning <var>promise</var>.
+    1. If {{SpeechRecognitionOptions/langs}} of <var>options</var> is an empty sequence, resolve <var>promise</var> with <code>false</code>, abort these steps, returning <var>promise</var>.
+    1. If the on-device speech recognition language pack for any <var>lang</var> in {{SpeechRecognitionOptions/langs}} of <var>options</var> is unsupported, resolve <var>promise</var> with <code>false</code>, abort these steps, returning <var>promise</var>.
     1. [=In parallel=], run the following steps:
         1. For each <var>lang</var> in {{SpeechRecognitionOptions/langs}} of <var>options</var>, initiate the download of the on-device speech recognition language pack for <var>lang</var> matching the requested {{SpeechRecognitionOptions/quality}} level floor.
             <p class=note>