When there was a server error that caused GoCardless API requests to fail, the GC library was failing to handle the error correctly due to a mistake in the function signature.

I was getting errors like this:

PHP Fatal error: Uncaught TypeError: Argument 1 passed to GoCardlessPro\RetryMiddlewareFactory::isRetryableServerError() must be an instance of GuzzleHttp\Psr7\Response, null given, called in vendor/gocardless/gocardless-pro/lib/RetryMiddlewareFactory.php on line 40 and defined in vendor/gocardless/gocardless-pro/lib/RetryMiddlewareFactory.php:88

The problem was that calling that method with an argument whose value is NULL is not allowed by the function signature.

This PR adds a ? before the type hint, which fixes the problem, the code is now valid and when circumstances arise (like network failures) the error can be properly handled and reported.