[RN] Fix setReceivedVideoQuality if we are not yet in a conference

It may happen that such action is fired while joining.

react/features/base/conference/middleware.js

 function _setReceiveVideoQuality({ dispatch, getState }, next, action) {
     const { audioOnly, conference } = getState()['features/base/conference'];
 
-    conference.setReceiverVideoConstraint(action.receiveVideoQuality);
-    if (audioOnly) {
-        dispatch(toggleAudioOnly());
+    if (conference) {
+        conference.setReceiverVideoConstraint(action.receiveVideoQuality);
+        if (audioOnly) {
+            dispatch(toggleAudioOnly());
+        }
     }
 
     return next(action);