Allow registering event handlers that do not accept any parameters

Originally reported by @fnesveda in [Slack](https://apify.slack.com/archives/CD0SF6KD4/p1732641589601389)

This :arrow_down: should work, now it does not:
```python
async def foo() -> None:
    print("Ding")

Actor.on(Event.bang, ding)
```

