Differentiate between `NaN` and `null` in the viewer

Currently, we don't do this and display and return in response `null` in both cases.
From the discussion in https://github.com/huggingface/dataset-viewer/pull/2797, this is agreed that it's important to let users know how to correctly treat data with these values.
This would require:
- [ ] Change in how we transform parquet to /first-rows and /rows response. I haven't figured out where exactly, but apparently `nan` values are somehow replaced with `null`.
- [ ] Change in response structure and field names in /statistics - for float columns add field `nan_count`, for other columns rename `nan_count` to `null_count` :/// (my bad with the original naming)