NoResponse stopped working

[chrysn]

This needs to be

• understood w/rt its implications for PlumbingRequest,
• fixed, and
• tested.

[therealprof]

I was just running into this while trying to send an EMPTY response. I get back an message saying:
WARNING:coap-server:Response does not carry response code (<Code 0 "EMPTY">), application probably violates protocol.

https://tools.ietf.org/html/rfc7252#section-5.2.2 clearly states that sending an EMPTY ACK is valid behaviour so I was looking into the code and found NoResponse and tried to use that instead which gave me this:

ERROR:coap-server:An exception occurred while rendering a resource: AttributeError("'Sentinel' object has no attribute 'code'")
Traceback (most recent call last):
  File "/usr/local/lib/python3.7/site-packages/aiocoap-0.4a1.post0-py3.7.egg/aiocoap/protocol.py", line 348, in _render_to_plumbing_request
    cancellation_future)
  File "/usr/local/lib/python3.7/site-packages/aiocoap-0.4a1.post0-py3.7.egg/aiocoap/protocol.py", line 465, in _render_to_plumbing_request_inner
    response = await self.serversite.render(request)
  File "/usr/local/lib/python3.7/site-packages/aiocoap-0.4a1.post0-py3.7.egg/aiocoap/resource.py", line 321, in render
    return await child.render(subrequest)
  File "/usr/local/lib/python3.7/site-packages/aiocoap-0.4a1.post0-py3.7.egg/aiocoap/resource.py", line 111, in render
    if response.code is None:
AttributeError: 'Sentinel' object has no attribute 'code'