if (SSL_IS_TLS13(s)) {
        int j;
       ...
        if (s->psk_server_callback != NULL) {
            for (j = 0; j < SSL_PKEY_NUM && !ssl_has_cert(s, j); j++);
            if (j == SSL_PKEY_NUM) {
                /* There are no certificates */
                prefer_sha256 = 1;
            }
        }

struct ssl_st {
    ...
    int error_code;
# ifndef OPENSSL_NO_PSK
    SSL_psk_client_cb_func psk_client_callback;
    SSL_psk_server_cb_func psk_server_callback;
# endif
   ...
};