Fix NestedDataStructure.data for empty dict
CONTRIBUTOR
I validated that this fixed the problem, thank you, @albertvillanova!

2021-03-03T20:58:17Z
https://github.com/huggingface/datasets/pull/1982#issuecomment-790050134
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still facing the same issue or similar:
from datasets import load_dataset
wtm14_test = load_dataset('wmt14',"de-en",cache_dir='./datasets')

~\.cache\huggingface\modules\datasets_modules\datasets\wmt14\43e717d978d2261502b0194999583acb874ba73b0f4aed0ada2889d1bb00f36e\wmt_utils.py in _split_generators(self, dl_manager)
    758         # Extract manually downloaded files.
    759         manual_files = dl_manager.extract(manual_paths_dict)
--> 760         extraction_map = dict(downloaded_files, **manual_files)
    761 
    762         for language in self.config.language_pair:

TypeError: type object argument after ** must be a mapping, not list
2021-03-04T14:10:04Z
https://github.com/huggingface/datasets/pull/1982#issuecomment-790644636
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Hi @sabania 
We released a patch version that fixes this issue (1.4.1), can you try with the new version please ?
```
pip install --upgrade datasets
```
2021-03-04T14:14:57Z
https://github.com/huggingface/datasets/pull/1982#issuecomment-790647832
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CONTRIBUTOR
I re-validated with the hotfix and the problem is no more.
2021-03-04T16:44:45Z
https://github.com/huggingface/datasets/pull/1982#issuecomment-790757599
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It's working. thanks a lot.
2021-03-04T16:46:04Z
https://github.com/huggingface/datasets/pull/1982#issuecomment-790758526
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Fix #1981