Fix use of empty credentials for HTTP and proxy authentication

Empty user and password fields are valid for HTTP and proxy
authentication. Thus it does not mean that the authentication
is cancelled. For informing the engine about the cancellation
of the authentication dialog make QAuthenticator instance null.

Change-Id: Iba1ce9d375b9b37c23f7a91fb583606d75d04af5
Reviewed-by: Kai Koehne <kai.koehne@theqtcompany.com>