fix(VideoLayout) make thumbnail iteration more resilient

If a failure occurs while we are still setting up the UI it's possible the local
thumbnail is still null, and none of the code assumes it may be null, so skip
it.

modules/UI/videolayout/VideoLayout.js

  */
 function getAllThumbnails() {
     return [
-        localVideoThumbnail,
+        ...localVideoThumbnail ? [ localVideoThumbnail ] : [],
         ...Object.values(remoteVideos)
     ];
 }