In some cases, e.g. when bad credentials are provided in an ftp URI,
QNetworkAccessAuthenticationManager::cacheCredentials is called with a
null authenticator. This authenticator should not be cached, because
it is useless, and leads to inconsistencies in the use of the cache
Task-number: QTBUG-40622
Change-Id: If2a0a422b915f268648f5eef1d68601446123371
Reviewed-by: Peter Hartmann <phartmann@blackberry.com>