[Checksums] If no configs exist prevent to run over empty list
Whoops I fixed it directly on master before checking that you have done it in this PR. We may close it Yeah, I saw :-) But I think we should add this as well since some datasets have an empty list of configs and then as the code is now it would fail. 

In this PR, I just make sure that the code jumps in the correct else if "there are no configs" as is the case for some datasets @mariamabarham  Sorry, I thought you meant a different commit . Just saw this one: https://github.com/huggingface/nlp/commit/7c644f284e2303b57612a6e7c904fe13906d893f
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All good then :-) 
`movie_rationales` e.g. has no configs.