Adding null typing to solve (latest) php 8.4 deprecation

[kevinoo]

Following PR #303

In the last PHP update to version 8.4, the declaration of variables initialized as null without proper typing was deprecated.

https://www.php.net/manual/en/migration84.deprecated.php

This PR intends to resolve the latest warning alert presented for the package within applications that use it.

Deprecated: Spatie\GoogleCalendar\Event::get(): Implicitly marking parameter $calendarId as nullable is deprecated, the explicit nullable type must be used instead in /var/www/html/vendor/spatie/laravel-google-calendar/src/Event.php on line 83

[freekmurze]

Thanks!