iOS null callback fix for setExternalUserId

Motivation: we want to make sure that the `setExternalUserId` works for iOS even if no callback is passed. Thus, we are re-adding the default callback but for iOS only. 

Note: for Android we want it to be nullable temporarily as a workaround for the current issue of the same callback firing multiple times (crashes) due to the `onSuccess` triggering once per channel.

Note: in the future, we intend to make argument non-nullability or nullability consistent across both platforms.

src/index.js

@@ -254,6 +254,10 @@ export default class OneSignal {
             return;
         }
 
+        if (!varArg2 && Platform.OS === 'ios') {
+            varArg2 = function(){};
+        }
+
         RNOneSignal.setExternalUserId(externalId, varArg1, varArg2);
     }