coap_mbedtls.c: Fix SNI hostname definition if not provided

MbedTLS based PKI clients may report

SSL - Attempt to verify a certificate without an expected hostname.
This is usually insecure.  In TLS clients, when a client authenticates a
server through its certificate, the client normally checks three things: ....

The application provided client_sni may have been removed as per
https://datatracker.ietf.org/doc/html/rfc6066#section-3

   Literal IPv4 and IPv6 addresses are not permitted in "HostName".

and so MbedTLS is now explicitly told that therw is no hostname.

src/coap_mbedtls.c

@@ -1435,9 +1435,7 @@ setup_client_ssl_session(coap_session_t *c_session,
       }
     }
 #endif /* MBEDTLS_SSL_SRV_C && MBEDTLS_SSL_ALPN */
-    if (m_context->setup_data.client_sni) {
-      mbedtls_ssl_set_hostname(&m_env->ssl, m_context->setup_data.client_sni);
-    }
+    mbedtls_ssl_set_hostname(&m_env->ssl, m_context->setup_data.client_sni);
 #if defined(MBEDTLS_SSL_PROTO_DTLS)
 #if MBEDTLS_VERSION_NUMBER >= 0x02100100
     mbedtls_ssl_set_mtu(&m_env->ssl, (uint16_t)c_session->mtu);