__bool__ method on Results

Wrote this code today: https://github.com/simonw/datasette-public/blob/1401bfae50e71c1dfd2bfb6954f2e86d5a7ab21b/datasette_public/__init__.py#L41
```python
    results = await db.execute(
        "select 1 from _public_tables where table_name = ?", [table_name]
    )
    if len(results):
        return True
```
Would be nice if I could use `if results` there instead.