[OAuth] Fix behaviour when ReplyHandler is not set

// Setter:
void QAbstractOAuth::setReplyHandler(QAbstractOAuthReplyHandler *handler);

// Getter:
QAbstractOAuthReplyHandler *QAbstractOAuth::replyHandler() const
{
    Q_D(const QAbstractOAuth);
    return d->replyHandler ? d->replyHandler.data() : d->defaultReplyHandler.data();
}

QObjectPrivate::connect(d->replyHandler.data() ... // nullptr if handler is not set
QAbstractOAuthReplyHandler *handler = replyHandler(); // default handler if handler not set
QObject::connect(reply, &QNetworkReply::finished, handler,

qt.core.qobject.connect: QObject::connect(QAbstractOAuthReplyHandler, QOAuth2AuthorizationCodeFlow): invalid nullptr parameter