Error when encoding a dataset with None objects with a Sequence feature
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This has been fixed by https://github.com/huggingface/datasets/pull/2900
We're doing a new release 1.12 today to make the fix available :)
There is an error when encoding a dataset with None objects with a Sequence feature

To reproduce:
```python
from datasets import Dataset, Features, Value, Sequence
data = {"a": [[0], None]}
features = Features({"a": Sequence(Value("int32"))})
dataset = Dataset.from_dict(data, features=features)
```
raises

```python
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-24-40add67f8751> in <module>
      2 data = {"a": [[0], None]}
      3 features = Features({"a": Sequence(Value("int32"))})
----> 4 dataset = Dataset.from_dict(data, features=features)
[...]
~/datasets/features.py in encode_nested_example(schema, obj)
    888         if isinstance(obj, str):  # don't interpret a string as a list
    889             raise ValueError("Got a string but expected a list instead: '{}'".format(obj))
--> 890         return [encode_nested_example(schema.feature, o) for o in obj]
    891     # Object with special encoding:
    892     # ClassLabel will convert from string to int, TranslationVariableLanguages does some checks

TypeError: 'NoneType' object is not iterable
```

Instead, if should run without error, as if the `features` were not passed
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