Dict feature is non-nullable while nested dict feature is
It looks like a bug, by default every feature should be nullable. I've linked a PR with a fix :) @mariosasko awesome thank you!
When i try to create a `Dataset` object with None values inside a dict column, like this: 
```python
from datasets import Dataset, Features, Value

Dataset.from_dict(
    {
        "dict": [{"a": 0, "b": 0}, None],
    }, features=Features(
        {"dict": {"a": Value("int16"), "b": Value("int16")}}
    )
)
```
i get `ValueError: Got None but expected a dictionary instead`.


At the same time, having None in _nested_ dict feature works, for example, this doesn't throw any errors:

```python
from datasets import Dataset, Features, Value, Sequence

dataset = Dataset.from_dict(
    {
        "list_dict": [[{"a": 0, "b": 0}], None],
        "sequence_dict": [[{"a": 0, "b": 0}], None],
    }, features=Features({
        "list_dict": [{"a": Value("int16"), "b": Value("int16")}],
        "sequence_dict": Sequence({"a": Value("int16"), "b": Value("int16")}),
    })
)
```
Other types of features also seem to be nullable (but I haven't checked all of them).

Version of `datasets` is the latest atm (2.18.0)


Is this an expected behavior or a bug?