RobertaTokenizer corner case with empty string
https://github.com/huggingface/transformers/blob/81484b447b7d8504ff5e1cfff38ec35918383963/src/transformers/tokenization_roberta.py#L239
this will introduce issue if `text == ""`, which will occur if anyone follows `run_glue.py` with QQP task as the `train.tsv` has two lines contains the empty column.
can be corrected to `if add_prefix_space and (not text or not text[0].isspace()):`
A PR is welcome!<|||||>created PR #3621 <|||||>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.
<|||||>cc @mfuntowicz @n1t0 

The problem is that the tokenizer does not allow empty strings to be passed (will lead to index out of bounds error when `text[0].isspace()`). An empty string is possible, according to OP, when using the QQP task which has such a format. OP added a PR here that you can have a look at https://github.com/huggingface/transformers/pull/3621<|||||>This issue speaks about fixing https://github.com/huggingface/transformers/blob/81484b447b7d8504ff5e1cfff38ec35918383963/src/transformers/tokenization_roberta.py#L239 which seems totally reasonable to me, but #3621 does a lot more than that, half of which I don't even understand.

@boy2000-007man could you update your PR to only fix the relevant line, and maybe add a test?<|||||>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.
