Fix deprecated warning in PHP 8

[olegisk]

Fix deprecation warning:
There's problem in class checking if have null instead of string:

        if (class_exists($this->getRequestService()->getResponseResourceFQCN())) {
            $responseResourceFCQN = $this->getRequestService()->getResponseResourceFQCN();
            $responseResource = $this->resourceFactory->createFromFqcn($serviceBaseName, $responseResourceFCQN, $data);
            $this->setResponseResource($responseResource);
            return;
        }

I suggest the follow solution:

        if ($this->getRequestService()->getResponseResourceFQCN() &&
            class_exists($this->getRequestService()->getResponseResourceFQCN())
        ) {
            $responseResourceFCQN = $this->getRequestService()->getResponseResourceFQCN();
            $responseResource = $this->resourceFactory->createFromFqcn($serviceBaseName, $responseResourceFCQN, $data);
            $this->setResponseResource($responseResource);
            return;
        }