net: lwm2m: Fix Notify response handling

As the CoAP library was refactored to handle response
messages as stated by RFC, the LwM2M message handling was
untouched.
LwM2M Notify handling is relying on response callback
to be called, but as per CoAP RFC an empty Ack is not
actually a response and the callback was not called anymore.

Fix the issue by calling the response callback when
we receive an empty ack for any confirmable response
messages.
This should not affect any handling of CoAP requests.

Signed-off-by: Seppo Takalo <seppo.takalo@nordicsemi.no>