Fix + Test
Fix an edge case of the blenderbot-90 tokenizer.

Closes #8029 

# Context

If the blenderbot-90 tokenizer is used to tokenize the following sequence:
```py
sequence = "Ok ."
```
It will split it in two tokens at first:
https://github.com/huggingface/transformers/blob/8bbe8247f13057b7df1b2c9abbfacb05b30020bf/src/transformers/tokenization_blenderbot.py#L221

Those two tokens will be `['Ok', '.']`

The issue is that, when passed the second token, the `bpe` method will convert it from `'.'` to `' .'` here:
https://github.com/huggingface/transformers/blob/8bbe8247f13057b7df1b2c9abbfacb05b30020bf/src/transformers/tokenization_blenderbot.py#L160

This then gets split on spaces here:
https://github.com/huggingface/transformers/blob/8bbe8247f13057b7df1b2c9abbfacb05b30020bf/src/transformers/tokenization_blenderbot.py#L166

This is where the issue lies, as it creates two strings: `["", "."]`, the first one being empty.

It then crashes a bit further as we try to index the empty string:
https://github.com/huggingface/transformers/blob/8bbe8247f13057b7df1b2c9abbfacb05b30020bf/src/transformers/tokenization_blenderbot.py#L171

## Proposal

Ensure that the token has a length > 0 before trying to manage it, otherwise ignore that token.

Added a test.


