Method replyTo should accept null as parameter

[jfradj]

Hello,

I think that the the method prototype isn't totally correct:

public function replyTo(string $message_id): self

Indeed, it should allow to set the value to null, so the prototype should be:

public function replyTo(?string $message_id): self

(note the question mark to all null to be passed as an argument). Right now, I used the empty string as a workaround, but it's a little bit ugly 🥲

I've audited the code, and everything has been designed to make that possible.