[CoAP] When resource is observed the QT CoAP client sends an acknowledgement packet which is not empty.

The Qt CoAP client sends an acknowledgement packet with CoAP code 0 which should be empty as per RFC 7252. (https://datatracker.ietf.org/doc/html/rfc7252#section-12.1). The CoAP server is treating this as a malformed packet.

This can be reproduced with the Qt example simplecoapclient and a coap server like the coapserver example in libcoap (https://github.com/obgm/libcoap).

To reproduce start the coapserver example and the simplecoapclient. Set the host as "::1" and resource as /time and start to observe. The server will discard the ack packages as malformed PDU and write a log to the output.