[Metric] Bertscore : Warning : Empty candidate sentence; Setting recall to be 0.
When running BERT-Score, I'm meeting this warning :

> Warning: Empty candidate sentence; Setting recall to be 0.

Code :

```
import nlp
metric = nlp.load_metric("bertscore")
scores = metric.compute(["swag", "swags"], ["swags", "totally something different"], lang="en", device=0)
```

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**What am I doing wrong / How can I hide this warning ?**