Fix crash on screen share

Don't try to wire up audio nodes if the stream has no audio track,
'cos it'll crash.

Fixes https://github.com/vector-im/element-call/issues/421

src/video-grid/useMediaStream.ts

@@ -202,7 +202,12 @@ export const useSpatialMediaStream = (
   const sourceRef = useRef<MediaStreamAudioSourceNode>();
 
   useEffect(() => {
-    if (spatialAudio && tileRef.current && !mute) {
+    if (
+      spatialAudio &&
+      tileRef.current &&
+      !mute &&
+      stream.getAudioTracks().length > 0
+    ) {
       if (!pannerNodeRef.current) {
         pannerNodeRef.current = new PannerNode(audioContext, {
           panningModel: "HRTF",