Validate the JSON data in load

Check to make sure that the loaded JSON actually contains data in the keys we are going to use before we use them, to avoid runtime exceptions on Nil.

lib/u2f/register_response.rb

@@ -20,6 +20,10 @@ def self.load_from_json(json)
         raise RegistrationError, code: data['errorCode']
       end
 
+      if data['clientData'].blank? || data['registrationData'].blank?
+        raise RegistrationError, code: 2
+      end
+
       instance = new
       instance.client_data_json =
         ::U2F.urlsafe_decode64(data['clientData'])