Issue: Transcript string 'null' converted to [None] by load_dataset()

## Issue
I am training a luxembourgish speech-recognition model in Colab with a custom dataset, including a dictionary of luxembourgish words, for example the speaken numbers 0 to 9. When preparing the dataset with the script 

`ds_train1 = mydataset.map(prepare_dataset)` 

the following error was issued:

```   
ValueError                                Traceback (most recent call last)
<ipython-input-69-1e8f2b37f5bc> in <module>()
----> 1 ds_train = mydataset_train.map(prepare_dataset)

11 frames
/usr/local/lib/python3.7/dist-packages/transformers/tokenization_utils_base.py in __call__(self, text, text_pair, add_special_tokens, padding, truncation, max_length, stride, is_split_into_words, pad_to_multiple_of, return_tensors, return_token_type_ids, return_attention_mask, return_overflowing_tokens, return_special_tokens_mask, return_offsets_mapping, return_length, verbose, **kwargs)
   2450         if not _is_valid_text_input(text):
   2451             raise ValueError(
-> 2452                 "text input must of type str (single example), List[str] (batch or single pretokenized example) "
   2453                 "or List[List[str]] (batch of pretokenized examples)."
   2454             )

ValueError: text input must of type str (single example), List[str] (batch or single pretokenized example) or List[List[str]] (batch of pretokenized examples).
```
Debugging this problem was not easy, all transcriptions in the dataset are correct strings. Finally I discovered that the transcription string 'null' is interpreted as [None] by the `load_dataset()` script. By deleting this row in the dataset the  training worked fine.

## Expected result: 
transcription 'null' interpreted as 'str' instead of 'None'.

## Reproduction
Here is the code to reproduce the error with a one-row-dataset.

```  
with open("null-test.csv") as f:
    reader = csv.reader(f)
    for row in reader:
        print(row)
```  

['wav_filename', 'wav_filesize', 'transcript']
['wavs/female/NULL1.wav', '17530', 'null']

```
dataset = load_dataset('csv', data_files={'train': 'null-test.csv'}) 
```  

Using custom data configuration default-81ac0c0e27af3514
Downloading and preparing dataset csv/default to /root/.cache/huggingface/datasets/csv/default-81ac0c0e27af3514/0.0.0/433e0ccc46f9880962cc2b12065189766fbb2bee57a221866138fb9203c83519...
Downloading data files: 100%
1/1 [00:00<00:00, 29.55it/s]
Extracting data files: 100%
1/1 [00:00<00:00, 23.66it/s]
Dataset csv downloaded and prepared to /root/.cache/huggingface/datasets/csv/default-81ac0c0e27af3514/0.0.0/433e0ccc46f9880962cc2b12065189766fbb2bee57a221866138fb9203c83519. Subsequent calls will reuse this data.
100%
1/1 [00:00<00:00, 25.84it/s]

```  
print(dataset['train']['transcript'])
``` 

[None]

## Environment info
```
!pip install datasets==2.2.2
!pip install transformers==4.19.2
```  

Comments:
Hi @mbarnig, thanks for reporting.

Please note that is an expected behavior by `pandas` (we use the `pandas` library to parse CSV files): https://pandas.pydata.org/docs/reference/api/pandas.read_csv.html
```
By default the following values are interpreted as NaN: 
‘’, ‘#N/A’, ‘#N/A N/A’, ‘#NA’, ‘-1.#IND’, ‘-1.#QNAN’, ‘-NaN’, ‘-nan’, ‘1.#IND’, ‘1.#QNAN’, ‘<NA>’, ‘N/A’, ‘NA’, ‘NULL’, ‘NaN’, ‘n/a’, ‘nan’, ‘null’.
```
(see "null" in the last position in the above list).

In order to prevent `pandas` from performing that automatic conversion from the string "null" to a NaN value, you should pass the `pandas` parameter `keep_default_na=False`:
```python
In [2]: dataset = load_dataset('csv', data_files={'train': 'null-test.csv'}, keep_default_na=False)
In [3]: dataset["train"][0]["transcript"]
Out[3]: 'null'
```

Thanks for the quick answer.

@albertvillanova I also ran into this issue, it had me scratching my head for a while! In my case it was tripped by a literal "NA" comment collected from a user-facing form (e.g., this question does not apply to me). Thankfully this answer was here, but I feel it is such a common trap that it deserves to be noted in the official docs, maybe [here](https://huggingface.co/docs/datasets/loading#csv)? 

I'm happy to submit a PR if you agree!