Properties of unset special tokens return the string 'None' in non verbose Tokenizers
### System Info

```shell
- `transformers` version: 4.20.0
- Platform: Linux-5.4.0-120-generic-x86_64-with-glibc2.27
- Python version: 3.8.0
- Huggingface_hub version: 0.7.0
- PyTorch version (GPU?): 1.11.0+cu102 (False)
- Tensorflow version (GPU?): not installed (NA)
- Flax version (CPU?/GPU?/TPU?): not installed (NA)
- Jax version: not installed
- JaxLib version: not installed
- Using GPU in script?: no
- Using distributed or parallel set-up in script?: no
```


### Who can help?

@SaulLu

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [ ] My own task or dataset (give details below)

### Reproduction

For example, `MarianTokenizer` does not set `bos_token`. The corresponding property returns the string `'None'`:

```python
>>> import transformers
>>> tokenizer = transformers.MarianTokenizer.from_pretrained("Helsinki-NLP/opus-mt-en-de", verbose=False)
>>> tokenizer.bos_token
'None'
```

### Expected behavior

```shell
The property should return None, not the string 'None'.
```

Hi @guillaumekln ,

Thanks a lot for letting us know about this issue! ~~I'm planning to fix it in the PR #17824~~ EDIT: sorry, I haven't seen your PR. Closing mine in favor of yours :hugs: <|||||>Thanks for the update! I also suggested a fix here #17797. Feel free to close this PR if needed.<|||||>Let's keep yours! 