Can radix_tree_update be slightly simplified?

**Issue by [mihaibudiu](https://github.com/mihaibudiu)**
_Thu Nov 24 19:46:23 2022_
_Originally opened as https://github.com/vmware/database-stream-processor/issues/238_

----

```
let agg = if input.key_valid() && input.key() == input_delta.key() {
aggregator.aggregate(&mut CursorGroup::new(&mut input, ()))
} else {
None
};
```

The spec of the aggregate function seems it to require None anyway for an empty input set.