Descriptive stats: wrong class label proportions when the labels are `-1` (no label)
e.g. for https://huggingface.co/datasets/glue/viewer/cola/test

the response contains

```json
{
    "column_name": "label",
    "column_type": "class_label",
    "column_statistics": {
        "nan_count": 0,
        "nan_proportion": 0.0,
        "n_unique": 1,
        "frequencies": {
            "acceptable": 1063
        }
    }
}
```

cc @polinaeterna 
Descriptive stats: wrong class label proportions when the labels are `-1` (no label): e.g. for https://huggingface.co/datasets/glue/viewer/cola/test

the response contains

```json
{
    "column_name": "label",
    "column_type": "class_label",
    "column_statistics": {
        "nan_count": 0,
        "nan_proportion": 0.0,
        "n_unique": 1,
        "frequencies": {
            "acceptable": 1063
        }
    }
}
```

cc @polinaeterna 