Ensure the reason is never an empty string, but None in HTTP responses

Otherwise, `aiohttp` (in tests) cannot parse the response with the trailing space in the status — `"HTTP/1.1 555 "` — since `"555 ".split()` returns only one element, and the whole string `"555 "` is used as the status with expectations of being a 3-digit numeric.

Signed-off-by: Sergey Vasilyev <nolar@nolar.info>

kopf/_kits/webhooks.py

@@ -214,13 +214,13 @@ async def _serve(
             response = await fn(data, webhook=webhook, sslpeer=sslpeer, headers=headers)
             return aiohttp.web.json_response(response)
         except admission.AmbiguousResourceError as e:
-            raise aiohttp.web.HTTPConflict(reason=str(e))
+            raise aiohttp.web.HTTPConflict(reason=str(e) or None)
         except admission.UnknownResourceError as e:
-            raise aiohttp.web.HTTPNotFound(reason=str(e))
+            raise aiohttp.web.HTTPNotFound(reason=str(e) or None)
         except admission.WebhookError as e:
-            raise aiohttp.web.HTTPBadRequest(reason=str(e))
+            raise aiohttp.web.HTTPBadRequest(reason=str(e) or None)
         except json.JSONDecodeError as e:
-            raise aiohttp.web.HTTPBadRequest(reason=str(e))
+            raise aiohttp.web.HTTPBadRequest(reason=str(e) or None)
 
     @staticmethod
     def _allocate_free_port() -> int: