Fix PHP 8.4 deprecations in bundled Phast SDK

[roboes]

Description

This PR resolves the PHP 8.4 deprecation warning: "Implicitly marking parameter as nullable is deprecated".

I have updated the function signatures within sdk/phast.php (and relevant classes) where a type hint is used with a null default value to use the explicit ?Type syntax.

Implementation Note

• These changes were applied to the bundled sdk/phast.php file to immediately resolve logging noise for WordPress users on modern PHP environments.
• A corresponding PR has also been submitted to the core kiboit/phast repository to address these at the source (Fix PHP 8.4 deprecation: explicit nullable type required kiboit/phast#21).

Compatibility

• PHP 7.4+ Compatible: Uses nullable syntax supported since PHP 7.1.
• Syntax Only: No changes to logic or functionality.
• Future-proof: Ensures clean logs for users on PHP 8.4+ without breaking older environments.