Don's ajust audio track if there is no audio track

On a M4 Mac Mini, there is no built-in microphone or
camera, so `getAudioTracks()` will always return an
empty array. In this case, we don't need to ajust
sampling rate to 48khz...

src/content/getusermedia/record/js/main.js

@@ -108,12 +108,14 @@ async function startRecording() {
   if (mimeType.split(';', 1)[0] === 'video/mp4') {
     // Adjust sampling rate to 48khz.
     const track = window.stream.getAudioTracks()[0];
-    const {sampleRate} = track.getSettings();
-    if (sampleRate != 48000) {
-      track.stop();
-      window.stream.removeTrack(track);
-      const newStream = await navigator.mediaDevices.getUserMedia({audio: {sampleRate: 48000}});
-      window.stream.addTrack(newStream.getTracks()[0]);
+    if (track) {
+      const {sampleRate} = track.getSettings();
+      if (sampleRate != 48000) {
+        track.stop();
+        window.stream.removeTrack(track);
+        const newStream = await navigator.mediaDevices.getUserMedia({audio: {sampleRate: 48000}});
+        window.stream.addTrack(newStream.getTracks()[0]);
+      }
     }
   }
   try {