[php-src] Issue #8712: SoapClient reflection return type missing for __call method in PHP 8.1

Issue: https://github.com/php/php-src/issues/8712 Author: sakarikl ### Description The following code: ```php <?php $reflection = new \ReflectionClass(\SoapClient::class); $method = $reflection->getMethod('__call'); var_dump($method->getReturnType()); ``` Resulted in this output: ``` NULL ``` But I expected this output instead: ``` object(ReflectionNamedType)#2022 (0) { } ``` reflection does not return anything as return type for __call method but in reality its return type is mixed. This causes real life problems if using ProxyManager for classes extending \SoapClient ### PHP Version 8.1.6 ### Operating System _No response_