Initialize MessageNotification properties

[angelomelonas]

Description

I have noticed the following error message when calling if ($messageNotification->context() !== null) {...} (for example) recently:

Uncaught PHP Exception Symfony\Component\Messenger\Exception\HandlerFailedException: "Handling "App\My\Class\Here" failed: Typed property Netflie\WhatsAppCloudApi\WebHook\Notification\MessageNotification::$context must not be accessed before initialization"

Perhaps there is a good reason why these properties are not initialized, so feel free to reject this pull request 😄

Thanks for the awesome SDK! ❤️

[avido]

Can this change also be released?

[aalbarca]

Can this change also be released?

Done.