[tls] Send an empty client certificate chain if we have no certificate

[tls] Send an empty client certificate chain if we have no certificate

RFC5246 states that "a client MAY send no certificates if it does not
have an appropriate certificate to send in response to the server's
authentication request".  This use case may arise when the server is
using optional client certificate verification and iPXE has not been
provided with a client certificate to use.

Treat the absence of a suitable client certificate as a non-fatal
condition and send a Certificate message containing no certificates as
permitted by RFC5246.

Reported-by: Alexandre Ravey <alexandre@voilab.ch>
Originally-implemented-by: Alexandre Ravey <alexandre@voilab.ch>
Signed-off-by: Michael Brown <mcb30@ipxe.org>