When using coap observe, server send notification in a confirmable or non confirmable way.
To stop the observation client should respond with a reset message.
But Reset messages are transmitted unreliably, the client must be prepared that its Reset messages aren't received by the server. (draft-ietf-core-observe)

Maybe client should always send a Reset Message to unexpected non-confirmable message.

I'm not sure but It seems that's what you try to do here.
But sendEmptyMessage seems to not send any RST message and exchange is always null.

(linked to leshan issue)