Streaming dataset looses `.feature` method after `.add_column`
### Describe the bug

After appending a new column to a streaming dataset using `.add_column`, we can no longer access the list of dataset features using the `.feature` method.

### Steps to reproduce the bug

```python
from datasets import load_dataset

original_dataset = load_dataset("librispeech_asr", "clean", split="validation", streaming=True)
print(original_dataset.features.keys())

# now add a new column to our streaming dataset
modified_dataset = original_dataset.add_column("new_column", ["some random text" for _ in range(50)])
print(modified_dataset.features.keys())
```
**Print Output:**
```
dict_keys(['file', 'audio', 'text', 'speaker_id', 'chapter_id', 'id'])
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
Cell In[1], line 8
      6 # now add a new column to our streaming dataset
      7 modified_dataset = original_dataset.add_column("new_column", ["some random text" for _ in range(50)])
----> 8 print(modified_dataset.features.keys())

AttributeError: 'NoneType' object has no attribute 'keys'
```

We see that we get the features for the original dataset, but not the modified one with the added column.


### Expected behavior

Features should be persevered after adding a new column, i.e. calling:
```python
print(modified_dataset.features.keys())
```

Should return:
```
dict_keys(['file', 'audio', 'text', 'speaker_id', 'chapter_id', 'id', 'new_column'])
```

### Environment info

- `datasets` version: 2.10.2.dev0
- Platform: Linux-4.19.0-23-cloud-amd64-x86_64-with-glibc2.28
- Python version: 3.9.16
- Huggingface_hub version: 0.13.3
- PyArrow version: 10.0.1
- Pandas version: 1.5.2