Make compatible with PHP 8.0

[s-damian]

Please, Is it possible to update the SVP package?
Thank you

[repat]

#671

[dmitryuk]

@Stephweb please close pull request

[nickfls]

@dev-and-web there's more in #696

and also, I was wondering, maybe replace the whole if block (in FormBuilder and FormAccessible) with?

return optional($this->model)->getFormValue($key);

equal use in:
/Eloquent/FormAccessible.php:52
/FormBuilder.php:1365

[nickfls]

Never mind; I see the code already moved on and this PR could be closed.
#696 however is not addressed