Avoid crash if session object is not ok (#146)

Add checks for accessToken and refresh token

Author: gustavoggs

ios/RNSpotifyRemoteConvert.m

@@ -205,7 +205,7 @@ +(id)SPTAppRemoteContentItems:(NSArray *) items{
 }
 
 +(id)SPTSession:(SPTSession *)session{
-    if(session == nil){
+    if(session == nil || session.accessToken == nil || session.refreshToken == nil){
         return [NSNull null];
     }