PHP 8.4 compatibility

[facine]

While preparing for PHP 8.4 compatibility, I encountered deprecation warnings related to implicitly nullable parameters. I've updated the code to fix these warnings while keeping backward compatibility with older PHP versions (e.g., 7.0). Let me know if you see any issues or prefer a different approach.

[xenophyle]

Thanks for bringing this up.

This page indicates that something like
public function __construct($authentication_settings, $http_options = [], ?MailchimpHttpClientInterface $client = NULL) {
is the preferred way to be compatible back to PHP 7.0, but let me know if it looks like I am misunderstanding the recommendation.

[facine]

The nullable type ?MailchimpHttpClientInterface is only valid since PHP 7.1.
And we are adding support to "php": ">=5.4.0", so we cannot use the ? nullable syntax at all.
For compatibility, omit the ? and handle null values in the method body or via PHPDoc.
Also note that MailchimpHttpClientInterface|null as a union type in a method signature is only supported since PHP 8.0; in earlier versions you can only declare this in PHPDoc.

[xenophyle]

Ok, got it, that makes sense.