Ensure /first-rows can always be created when truncating (ie: avoid TooBigContentError) 
See https://huggingface.co/datasets/mikehemberger/inat_2021_train_mini_plantae

```
The size of the content of the first rows (358268 B) exceeds the maximum supported size (200000 B) even after truncation. Please report the issue.

Error code:   TooBigContentError
```

Initially reported in https://github.com/huggingface/datasets-server/issues/1957, but the issue is somewhat different.
Ensure /first-rows can always be created when truncating (ie: avoid TooBigContentError) : See https://huggingface.co/datasets/mikehemberger/inat_2021_train_mini_plantae

```
The size of the content of the first rows (358268 B) exceeds the maximum supported size (200000 B) even after truncation. Please report the issue.

Error code:   TooBigContentError
```

Initially reported in https://github.com/huggingface/datasets-server/issues/1957, but the issue is somewhat different.