fix: do not add the key to the hash if its value is nil

In Rails, `nil` responds to `as_json` therefore the attributes whose
value is `nil` are added to the result of `as_json`, which may end up
causing issues with some authenticators – see #441.

This commits adds another condition to the if in order to prevent
this from happening.

lib/webauthn/json_serializer.rb

@@ -13,7 +13,7 @@ def to_hash_with_camelized_keys
       attributes.each_with_object({}) do |attribute_name, hash|
         value = send(attribute_name)
 
-        if value.respond_to?(:as_json)
+        if value && value.respond_to?(:as_json)
           hash[camelize(attribute_name)] = value.as_json
         elsif value
           hash[camelize(attribute_name)] = deep_camelize_keys(value)