fix: guard for cases where user hits leave before being connected to the room

Co-authored-by: Tudor-Ovidiu Avram <tudor.potecaru@8x8.com>

conference.js

     leaveRoomAndDisconnect() {
         APP.store.dispatch(conferenceWillLeave(room));
 
-        if (room.isJoined()) {
+        if (room && room.isJoined()) {
             return room.leave().then(disconnect, disconnect);
         }