Encourage Using `if not ...` Instead of `if ... is None` when Applicable

For example:
```python
def plugin(self, key: str, plugin: Plugin | None = None) -> Plugin | None:
    if plugin is None:
        return self._plugins[key]
    self._plugins[key] = plugin
    plugin.on_load(self)
```
`plugin` is never treated as `False` when meaningful. It should be simply `if not plugin`.