fix deprecation error on PHP 8.4

[tenzap]

Implicitly marking parameter $headers as nullable is deprecated, the explicit nullable type must be used instead on line 117 in file vendor/datto/json-rpc-http/src/Client.php

[RLHawk1]

I contacted the original developer of this, and he doesn't have access to merge any pull requests. So my company has forked this and plans to maintain it going forward. You can use it at https://github.com/hcs-llc/php-json-rpc