send RDP_NEG_REQ also in the case of a null server certificate

In the case of an RDP server with security set to "RDP" level and null server certificate, the negotiations will return error code SSL_NOT_ALLOWED_BY_SERVER.  In that case, we still want to send the RDP_NEG_REQ negotiation request -- otherwise things like multimonitor layout cannot be passed to the server.   This results in Span Mode rather than True Multimonitor Mode (and probably some other effects as well).