fix(rn,navigation) fix navigating back to the welcome page

The CONFERENCE_WILL_LEAVE reducer in base/conference wipes the state so
we cannot rely on the old room value.

We may want to revisit this in the future.

react/features/mobile/navigation/middleware.js

@@ -38,7 +38,7 @@ function _setRoom(store, next, action) {
 
     if (!oldRoom && newRoom) {
         navigateRoot(screen.conference.root);
-    } else if (oldRoom && !newRoom) {
+    } else if (!newRoom) {
         navigateRoot(screen.root);
     }