dtls13: stop creating malformed certificate_authorities extension

[tyhik]

DTLS 1.3 Dimpl server handshake with BoringSSL client fails because Dimpl creates malformed certificate_authorities extension.

According to RFC 8446 4.2.4, this extension is optional, but if present, it must not be empty. Here's the RFC's definition of it:

  opaque DistinguishedName<1..2^16-1>;
  struct {
      DistinguishedName authorities<3..2^16-1>;
  } CertificateAuthoritiesExtension;

Note the minimum size 3 of authorities.

[algesten]

This is a repeat of #153