bug: DBSF reranker ZeroDivisionError

[pavelm10]

DBSF reranker crashes with ZeroDivisionError when number of results is 1

def normalize(response: List[models.ScoredPoint]) -> List[models.ScoredPoint]:
        total = sum([point.score for point in response])
        mean = total / len(response)
>       variance = sum([(point.score - mean) ** 2 for point in response]) / (len(response) - 1)
E       ZeroDivisionError: float division by zero