ArrowInvalid when mapping dataset with missing values
Hi ! It fails because of the feature type inference.

Because the first 1000 examples all have null values in the "match" field, then it infers that the type for this field is `null` type before writing the data on disk. But as soon as it tries to map an example with a non-null "match" field, then it fails.

To fix that you can either:
- increase the writer_batch_size to >2000 (default is 1000) so that some non-null values will be in the first batch written to disk
```python
datasets = datasets.map(lambda e: {'labels': e['match']}, remove_columns=['id'], writer_batch_size=2000)
```
- OR force the feature type with:
```python
from datasets import Features, Value

features = Features({
    'conflict': Value('int64'),
    'date': Value('string'),
    'headline': Value('string'),
    'match': Value('float64'),
    'label': Value('float64')
})
datasets = datasets.map(lambda e: {'labels': e['match']}, remove_columns=['id'], features=features)
```
## Describe the bug
I encountered an `ArrowInvalid` when mapping dataset with missing values. 
Here are the files for a minimal example. The exception is only thrown when the first line in the csv has a missing value (if you move the last line to the top it isn't thrown).
[data_small.csv](https://github.com/huggingface/datasets/files/7037838/data_small.csv)
[data.csv](https://github.com/huggingface/datasets/files/7037842/data.csv)

## Steps to reproduce the bug
```python
from datasets import load_dataset

datasets = load_dataset("csv", data_files=['data_small.csv'])

datasets = datasets.map(lambda e: {'labels': e['match']},
                        remove_columns=['id'])
```

## Expected results
No error

## Actual results
```
File "pyarrow/error.pxi", line 84, in pyarrow.lib.check_status
pyarrow.lib.ArrowInvalid: Invalid null value
```

## Environment info
- `datasets` version: 1.5.0
- Platform: Linux-5.11.0-25-generic-x86_64-with-glibc2.29
- Python version: 3.8.10
- PyTorch version (GPU?): 1.7.1+cpu (False)
- Tensorflow version (GPU?): 2.4.1 (False)
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no
