Fix None issue with Sequence of dict
`Features.encode_example` currently fails if it contains a sequence if dict like `Sequence({"subcolumn": Value("int32")})` and if `None` is passed instead of the dict.

```python
  File "/Users/quentinlhoest/Desktop/hf/datasets/src/datasets/features/features.py", line 1310, in encode_example
    return encode_nested_example(self, example)
  File "/Users/quentinlhoest/Desktop/hf/datasets/src/datasets/features/features.py", line 973, in encode_nested_example
    return {k: encode_nested_example(sub_schema, sub_obj) for k, (sub_schema, sub_obj) in zip_dict(schema, obj)}
  File "/Users/quentinlhoest/Desktop/hf/datasets/src/datasets/features/features.py", line 973, in <dictcomp>
    return {k: encode_nested_example(sub_schema, sub_obj) for k, (sub_schema, sub_obj) in zip_dict(schema, obj)}
  File "/Users/quentinlhoest/Desktop/hf/datasets/src/datasets/features/features.py", line 998, in encode_nested_example
    for k, (sub_schema, sub_objs) in zip_dict(schema.feature, obj):
  File "/Users/quentinlhoest/Desktop/hf/datasets/src/datasets/utils/py_utils.py", line 207, in zip_dict
    yield key, tuple(d[key] for d in dicts)
  File "/Users/quentinlhoest/Desktop/hf/datasets/src/datasets/utils/py_utils.py", line 207, in <genexpr>
    yield key, tuple(d[key] for d in dicts)
TypeError: 'NoneType' object is not subscriptable
```

I fixed this issue and updated the tests (this case was missing in the tests)
_The documentation is not available anymore as the PR was closed or merged._ Merging since I'd like do do a patch release soon for this one