Roberta Tokenizer crashes when tokenizing  empty string in 2.8.0
```
import transformers

tokenizer = transformers.AutoTokenizer.from_pretrained("roberta-large")
tokenzer.tokenize("")
```

  File "/scratch/wh629/nlu/env/lib/python3.7/site-packages/transformers/tokenization_utils.py", line 1054, in encode_plus
    first_ids = get_input_ids(text)
  File "/scratch/wh629/nlu/env/lib/python3.7/site-packages/transformers/tokenization_utils.py", line 1028, in get_input_ids
    tokens = self.tokenize(text, add_special_tokens=add_special_tokens, **kwargs)
  File "/scratch/wh629/nlu/env/lib/python3.7/site-packages/transformers/tokenization_utils.py", line 759, in tokenize
    text = self.prepare_for_tokenization(text, **kwargs)
  File "/scratch/wh629/nlu/env/lib/python3.7/site-packages/transformers/tokenization_roberta.py", line 239, in prepare_for_tokenization
    if add_prefix_space and not text[0].isspace():
IndexError: string index out of range
This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>Fixed by #4209