Fix PHP 8.4 deprecation: Implicitly marking parameter as nullable is deprecated

[toddb02]

[2025-07-28 21:20:09] local.WARNING: Sendbird\Api\UserApi::__construct(): Implicitly marking parameter $client as nullable is deprecated, the explicit nullable type must be used instead in vendor/sendbird/sendbird-platform-sdk-php/lib/Api/UserApi.php on line 79

Please fix all type-hinted method parameters accordingly.

public function __construct(
    ClientInterface $client = null,
    Configuration $config = null,
    HeaderSelector $selector = null,
    $hostIndex = 0
)...

becomes

public function __construct(
    ?ClientInterface $client = null,
    ?Configuration $config = null,
    ?HeaderSelector $selector = null,
    $hostIndex = 0
)...

This floods the logs with deprecations warnings.