Can't align optional features of struct
Friendly bump, I would be happy to work on this issue once I get the go-ahead from the dev team.  Thanks for the PR!

I'm struggling with this as well and would love to see this PR merged. My case is slightly different, with keys completely missing rather than being `None`:

```
ds = Dataset.from_dict({'speaker': [{'name': 'Ben'}]})
ds2 = Dataset.from_dict({'speaker': [{'name': 'Fred', 'email': 'abc@aol.com'}]})
print(concatenate_datasets([ds, ds2]).features)
print(concatenate_datasets([ds, ds2]).to_dict())
```

I would expect this to work as well because other Dataset functions already handle this situation well. For example, this works just as expected:

```
ds = Dataset.from_dict({'n': [1,2]})
ds_mapped = ds.map(lambda x: {
    'speaker': {'name': 'Ben'} if x['n'] == 1 else {'name': 'Fred', 'email': 'abc@aol.com'}
})
print(ds_mapped)
```
### 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.