I run a homeserver whose logs show a high number of incoming empty
presence EDUs originating from the user agent "Conduwuit/0.4.4". They
arrive at a rate of about 2 queries per second per Conduwuit server.
The empty EDUs all look the same, only with `origin_server_ts`
increasing:
```
{"origin":"example.com","origin_server_ts":1720266475601,"edus":[{"edu_type":"m.presence","content":{"push":[]}}]}
```
These updates are unnecessary because they don't do anything. They
only increase network traffic and CPU usage on both sides.
After this commit, the empty presence updates are no longer inserted
into the outgoing event queue.
2bc53139fa