PHP 8.4 implicit nullable deprecation

Problem/Motivation

When using PHP 8.4 this module is triggering a "use explicit nullable type" deprecation.

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elasticsearch_helper/src/Elasticsearch/Index/FieldDefinition.php
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Deprecated: Drupal\elasticsearch_helper\Elasticsearch\Index\FieldDefinition::toArray(): Implicitly marking parameter $source as nullable is deprecated, the explicit nullable type must be used instead
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elasticsearch_helper/src/Plugin/ElasticsearchIndexBase.php
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Deprecated: Drupal\elasticsearch_helper\Plugin\ElasticsearchIndexBase::dispatchOperationErrorEvent(): Implicitly marking parameter $request_wrapper as nullable is deprecated, the explicit nullable type must be used instead in 
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Proposed resolution

Use Implicitly nullable parameter as requested by PHP 8.4

Remaining tasks

• make a patch
• review
• merge

User interface changes

None

API changes

None

Data model changes

None