Channel post. AttributeError: 'NoneType' object has no attribute 'username'

When sending message in channel, there are only update_id and channel_post params. No user.

```
web_protocol.py :393 log_exception() Error handling request
Traceback (most recent call last):
File "/usr/local/lib/python3.9/site-packages/aiohttp/web_protocol.py", line 422, in _handle_request
resp = await self._request_handler(request)
File "/usr/local/lib/python3.9/site-packages/aiohttp/web_app.py", line 499, in _handle
resp = await handler(request)
File "/home/app/codexbot/lib/server.py", line 16, in wrapper
result = await function(self, {
File "/home/app/codexbot/services/telegram/telegram.py", line 55, in telegram_callback
await self.send_message_to_app(update)
File "/home/app/codexbot/services/telegram/telegram.py", line 65, in send_message_to_app
if update.message.user.username:
AttributeError: 'NoneType' object has no attribute 'username'
```