fix: account for possible -1 value in ClassLabel feature in statistics computation
will fix https://github.com/huggingface/datasets-server/issues/1833

It appeared that `ClassLabel` feature might have `-1` (int) value which means "no label", I didn't know that so I end up having the last value of `ClassLabel.names` instead of it lol.

What would be the best way to represent it? It's not a null value. Now I convert it to string form (like `"-1"`). I thought of converting it to something like `"no_label"` or `"(no label)"` (as the viewer currently displays them in rows) but in theory it might match to the existing label specified in `ClassLabel.names` explicitly. Here is an example of response:

```python
 {
        "nan_count": 0,
        "nan_proportion": 0.0,
        "n_unique": 2,  # num unique values stays the same as when there is no `no label` values
        "frequencies": {
        	"cat": 10
        	"dog": 5,
        	"-1": 10,
        }
    }
```
Alternatively, I can add a new field "num_no_labels" to store this number instead of having it as a key in "frequencies".

wdyt?
fix: account for possible -1 value in ClassLabel feature in statistics computation: will fix https://github.com/huggingface/datasets-server/issues/1833

It appeared that `ClassLabel` feature might have `-1` (int) value which means "no label", I didn't know that so I end up having the last value of `ClassLabel.names` instead of it lol.

What would be the best way to represent it? It's not a null value. Now I convert it to string form (like `"-1"`). I thought of converting it to something like `"no_label"` or `"(no label)"` (as the viewer currently displays them in rows) but in theory it might match to the existing label specified in `ClassLabel.names` explicitly. Here is an example of response:

```python
 {
        "nan_count": 0,
        "nan_proportion": 0.0,
        "n_unique": 2,  # num unique values stays the same as when there is no `no label` values
        "frequencies": {
        	"cat": 10
        	"dog": 5,
        	"-1": 10,
        }
    }
```
Alternatively, I can add a new field "num_no_labels" to store this number instead of having it as a key in "frequencies".

wdyt?