ValueError on Windows when loading a repo card with empty YAML

[lhoestq]

For example with a dataset card like this

---
---
# Begin of markdown

Some cool dataset card

It fails on windows with ValueError, while on macos and linux we can load the repo card and get and empty DatasetCardData.
This is because the regex used does find a match but it's empty. Therefore yaml.safe_load returns None and we get a ValueError since its not a dict as expected.

ValueError: repo card metadata block should be a dict

This is not super important but makes our CI fail at datasets (I'll just skip a test for now)

[Wauplin]

Thanks for reporting @lhoestq . Let's make a quick fix even though I'll not make a hot-fix release just for this one.

[lhoestq]

yea it's fine, thanks

[Wauplin]

Closed by #1547.