PHP 8.4 Deprecation warnings

[PNardman]

Feature or problem being solved

Adding explicit nullable to parameters since implicit nullable parameters are deprecated in PHP 8.4

Affected code:

Exportable.php
FastExcel.php
Importable.php

Further details

Adding the explicit nullable parameter, gets rid of unnecessary deprecation warnings for php 8.4.

Links / references

Implicitly nullable values are deprecated in php 8.4.

See:
https://php.watch/versions/8.4/implicitly-marking-parameter-type-nullable-deprecated

Seems there is an open pull request:

#363

[OtavioMoreira]

Do we have any idea when we will implement this? These are specific adjustments