PHP 8.4 - Implicitly nullable via default value null deprecation

Problem/Motivation

The module has PHP 8.4 deprecation in the form of "Implicitly nullable via default value null" - https://wiki.php.net/rfc/deprecate-implicitly-nullable-types

This produces deprecation warnings from PHP.

Steps to reproduce

Install Drupal 11+ and the 1.0.0-beta5 version of the feeds_ex module running on PHP 8.4 and observe the deprecation warnings.

Proposed resolution

See attached patch.

Remaining tasks

User interface changes

API changes

Data model changes