Do not remove None values in RepoCardData serialization

[Wauplin]

More details in huggingface/datasets#7243 (comment) and especially huggingface/datasets#7243 (comment).

When serializing repo cards metadata, we are currently removing any none values from any dictionary, set, list or tuple. This is done since the module has been introduced in #940 and never caused problems until huggingface/datasets#7243 was reported. I do find automatically removing None values inside values to be pretty clunky as it is values set by the user and not default ones.

This PR updates this behavior to remove only top-level None values, i.e. values that are None by default and that therefore shouldn't be serialized (as not set by the users themselves). The _remove_none helper is still kept to remove None values from the model-index (where it's rightfully necessary), but that's all.

⚠️ this is a breaking change. In practice, I don't expect it to break anything in the wild.

[HuggingFaceDocBuilderDev]

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[hanouticelina]

LGTM! ruff formatting tests/test_repocard_data.py + the fixes made in #2621 should make the CI green ✅

[Wauplin]

Thanks for the quick reviews :)

[julien-c]

change makes sense to me 👍

[alex-hh]

Thanks for fixing this! - any plans for a new release with the fix?

[Wauplin]

This fix will be shipped in next release but can't commit on when it'll be. In the meantime, I advice you to install huggingface_hub from source: https://huggingface.co/docs/huggingface_hub/installation#install-from-source.