Fix PHP 8.4 deprecation: Implicitly marking parameter as nullable is deprecated, the explicit nullable type must be used instead

[jhellemeister-tb]

Here is a list of the locations (by our usage) where the deprecation warning on "implicitly marking parameter" for the HubSpot library is located.

File path	Line
HubSpot\Client\Crm\Objects\Model\SimplePublicObjectInputForCreate	251
HubSpot\Client\Crm\Objects\Model\Error	281
HubSpot\Client\Crm\Objects\Model\SimplePublicObject	287
HubSpot\Client\Crm\Objects\Model\SimplePublicObjectInput	245
HubSpot\Client\Crm\Objects\Api\BasicApi	97

[brianlmoon]

Those of us migrating to PHP 8.4 needs this sooner rather than later if possible. Would a pull request be accepted?