Can't align optional features of struct
### Describe the bug

Hello!

I'm currently experiencing an issue where I can't concatenate datasets if an inner field of a Feature is Optional.

I have a column named `speaker`, and this holds some information about a speaker.

```python
@dataclass
class Speaker:
   name: str
   email: Optional[str]
```

If I have two datasets, one happens to have `email` always None, then I get `The features can't be aligned because the key email of features`




### Steps to reproduce the bug

You can run the following script:

```python

ds = Dataset.from_dict({'speaker': [{'name': 'Ben', 'email': None}]})
ds2 = Dataset.from_dict({'speaker': [{'name': 'Fred', 'email': 'abc@aol.com'}]})
concatenate_datasets([ds, ds2])

>>>The features can't be aligned because the key speaker of features {'speaker': {'email': Value(dtype='string', id=None), 'name': Value(dtype='string', id=None)}} has unexpected type - {'email': Value(dtype='string', id=None), 'name': Value(dtype='string', id=None)} (expected either {'email': Value(dtype='null', id=None), 'name': Value(dtype='string', id=None)} or Value("null").

```

### Expected behavior

I think this should work; if two top-level columns were in the same situation it would properly cast to `string`.

```python
ds = Dataset.from_dict({'email': [None, None]})
ds2 = Dataset.from_dict({'email': ['abc@aol.com', 'one@yahoo.com']})
concatenate_datasets([ds, ds2])
>>> # Works!
```

### Environment info

- `datasets` version: 2.15.1.dev0
- Platform: Linux-5.15.0-89-generic-x86_64-with-glibc2.35
- Python version: 3.9.13
- `huggingface_hub` version: 0.19.4
- PyArrow version: 9.0.0
- Pandas version: 1.4.4
- `fsspec` version: 2023.6.0

I would be happy to fix this issue.