fix(JitsiTrack): set video element src to empty string instead of null on #detach()

Setting the `src` of a video element to null triggered a network request for resource "null".

modules/RTC/JitsiTrack.js

     {
         if(!container)
         {
-            require("./RTCUtils").setVideoSrc(this.containers[i], null);
+            require("./RTCUtils").setVideoSrc(this.containers[i], '');
         }
         if(!container || $(this.containers[i]).is($(container)))
         {
     }
 
     if(container) {
-        require("./RTCUtils").setVideoSrc(container, null);
+        require("./RTCUtils").setVideoSrc(container, '');
     }
 }