fix(conference): do not go back to video

Do not go back to video from screen sharing if there was no video stream
at the time when screen sharing was being started.

conference.js

         let externalInstallation = false;
 
         if (shareScreen) {
+            const didHaveVideo = Boolean(this.localVideo);
+
             return createLocalTracks({
                 desktopSharingSources: options.desktopSharingSources,
                 devices: ['desktop'],
                 // close external installation dialog on success.
                 if(externalInstallation)
                     $.prompt.close();
-                stream.on(
-                    TrackEvents.LOCAL_TRACK_STOPPED,
-                    () => {
-                        // if stream was stopped during screensharing session
-                        // then we should switch to video
-                        // otherwise we stopped it because we already switched
-                        // to video, so nothing to do here
-                        if (this.isSharingScreen) {
-                            this.toggleScreenSharing(false);
+                if (didHaveVideo) {
+                    stream.on(
+                        TrackEvents.LOCAL_TRACK_STOPPED,
+                        () => {
+                            // If the stream was stopped during screen sharing
+                            // session then we should switch back to video.
+                            if (this.isSharingScreen) {
+                                this.toggleScreenSharing(false);
+                            }
                         }
-                    }
-                );
+                    );
+                }
                 return this.useVideoStream(stream);
             }).then(() => {
                 this.videoSwitchInProgress = false;