Remove all paperswithcode_id: null
On the Hub there is a validation error on the `paperswithcode_id` tag when the value is `null`:

<img width="686" alt="image" src="https://user-images.githubusercontent.com/42851186/177151825-93d341c5-25bd-41ab-96c2-c0b516d51c68.png">

We've been using `null` to specify that we checked on pwc but the dataset doesn't exist there.

To have the validation working again we can simply remove all the `paperswithcode_id: null`.

cc @julien-c 