Fix PHP 8.4 deprecated implicit nullable parameter

[HCHABANE]

When using the getbrevo/brevo-php SDK with PHP 8.4, deprecation warnings are triggered due to constructor parameters being implicitly marked as nullable without using the explicit nullable type (?). PHP 8.4 now requires that any parameter with a default value of null must be explicitly declared as nullable in the type hint.

brevo-php-php84-clean.patch

[Benjamin-K]

This duplicates #57