When I try to do the following, I got error:
```python
ds = Dataset.from_dict(
{
"question": ["question"],
"answer": ["answer"],
"contexts": [["context"]],
}
)
from ragas import evaluate
from ragas.metrics import Faithfulness
evaluate(dataset =ds, metrics=[Faithfulness(batch_size=1)])
```
```
KeyError: "Column ground_truths not in the dataset. Current columns in the dataset: ['question', 'answer', 'contexts']"
```
But `ground_truths ` is not needed for `Faithfulness` .
This PR is to fix it.