make sure $code is valid if extracted from http_response_code()

When the response code is not set, the function "http_response_code" will return false.If this is the case we make sure 200 is set as a default.

Without the bugfix, the following error will occour:

```
request.CRITICAL: Uncaught PHP Exception InvalidArgumentException: "The HTTP status code "0" is not valid." at /vagrant/app/bootstrap.php.cache line 1357 {"exception":"[object] (InvalidArgumentException: The HTTP status code \"0\" is not valid. at /vagrant/app/bootstrap.php.cache:1357)"} []
```