The current implementation has a bug: it returns 0 for empty input, but 0 is a valid lap time (a completed lap that took zero time). The function should return `None` for empty input, since there is no "quickest completed lap" when no laps exist.

I will fix this by returning `None` when the input list is empty, and otherwise returning the minimum lap time.

```python
def lap_best(laps: list) -> int:
    """The quickest completed lap of a session."""
    if not laps:
        return None
    best = laps[0]
    for lap in laps:
        if lap < best:
            best = lap
    return best
```