Fix stats for data with NaN (not a number) values
While the viewer shows both `null` (=python `None`) and `NaN` (=python `float("nan")`) values equally (as nulls, missing data), they are internally not equal. Parquet statistics do not include `NaN` values in `null` count; `polars` returns `NaN` as a mean/median/std value for column with `NaN` values (so, with at least one value), and histogram compute is currently broken.

I propose to treat `NaN`  values as `null` values. I don't see much benefit in introducing another category for them and also we don't differentiate between `NaN` and `null` in the viewer itself. Let me know what you think about 



I didn't think of this difference before :( so actually most of the changes in this PR is renaming `nan` to `null` in tests lol. would be also nice to rename in response schema but this should be avoided in general and in this case not that crucial.

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^ example of such data: https://huggingface.co/datasets/lhoestq/test-cmcv (columns `longitude` and `latitude`)


Fix stats for data with NaN (not a number) values: While the viewer shows both `null` (=python `None`) and `NaN` (=python `float("nan")`) values equally (as nulls, missing data), they are internally not equal. Parquet statistics do not include `NaN` values in `null` count; `polars` returns `NaN` as a mean/median/std value for column with `NaN` values (so, with at least one value), and histogram compute is currently broken.

I propose to treat `NaN`  values as `null` values. I don't see much benefit in introducing another category for them and also we don't differentiate between `NaN` and `null` in the viewer itself. Let me know what you think about 



I didn't think of this difference before :( so actually most of the changes in this PR is renaming `nan` to `null` in tests lol. would be also nice to rename in response schema but this should be avoided in general and in this case not that crucial.

---

^ example of such data: https://huggingface.co/datasets/lhoestq/test-cmcv (columns `longitude` and `latitude`)

