Fix PHP 8.4 deprecation

[tivvie]

PHP message: PHP Deprecated:
MailerLite\LaravelElasticsearch\Manager::connection(): Implicitly marking parameter $name as nullable is deprecated, the explicit nullable type must be used instead

Changelog entries:
https://www.php.net/manual/en/migration84.deprecated.php

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before submitting your pull request, and that you have:

• provided a rationale for your change (I try not to add features that are going to have a limited user-base)
• used the PSR-2 Coding Standard
• added tests
• documented any change in behaviour (e.g. updated the README.md, etc.)
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Thank you!

[gord007]

It's OK, pls merge