PHP 8.4 deprecation: implicit nullable parameter in dx_toolkit PluginManager

Problem/Motivation

PHP 8.4 has deprecated implicit nullable types, causing deprecation warnings in the dx_toolkit module. These warnings need to be addressed to ensure compatibility with future PHP versions and to maintain clean error logs.

Steps to reproduce

Install and enable the dx_toolkit module
Use PHP 8.4
Clear Drupal cache (e.g., using `drush cr`)
Observe the following deprecation warning:
Drupal\dx_toolkit\Plugin\PluginManager::createInstances(): Implicitly marking parameter $plugin_ids as nullable is deprecated, the explicit nullable type must be used instead

Proposed resolution

Explicitly declare nullable parameters in PluginManager::createInstances()

Remaining tasks

Test the changes to ensure they resolve the deprecation warning
Verify that the module's functionality remains intact after the changes

User interface changes

None

API changes

None

Data model changes

None