EAP-TLS peer: Handle possible application data at the end

EAP-TLS peer: Handle possible application data at the end

EAP-TLS with TLS 1.3 uses an empty application data record from the
server to indicate end of the exchange, so EAP-TLS peer will need to
check for this special case and finish the exchange with an empty
EAP-TLS (ACK) so that the server can send out EAP-Success.

Signed-off-by: Jouni Malinen <j@w1.fi>