[Schema] Omit the id of an error response that never had one (#442)

* [Schema] Omit the id of an error response that never had one

An error response whose id could not be read sent `"id": ""`, which claims
the peer issued a request with an empty-string id - a different statement
from "the id could not be read". JSON-RPC spells the latter as a null id,
and a receiver that cannot read the id is exactly the case the code covers.

[BC Break] Error::$id accepts null and getId() may return it. All the for*()
factories default to null, fromArray() accepts a missing or explicitly-null
id, and the member is omitted from the serialized form when absent.

* Guard null response id in Client/Server Protocol handlers

Also fix Error::jsonSerialize()'s phpdoc: data belongs inside error, not beside it.

src/Server/Protocol.php

@@ -341,6 +341,12 @@ class Protocol
 
         $messageId = $response->getId();
 
+        if (null === $messageId) {
+            $this->logger->warning('Received an id-less error response from client; cannot correlate it to a pending request.', ['response' => $response->jsonSerialize()]);
+
+            return;
+        }
+
         $session->set(self::SESSION_RESPONSES.".{$messageId}", $response->jsonSerialize());
         $session->forget(self::SESSION_ACTIVE_REQUEST_META);