Fixed inconsistency in several fast tokenizers
Author: Towdo
Pull request: yes

Fixed case where behavior of BertTokenizer and BertTokenizerFast is different.
An empty list will be evaluated to `False` but not to `is None`.

(I mistakenly closed my first merge request)

Fixes #26123


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Comments:
> Thanks a lot for these changes! Let's add a small test in the test_tokenization_common 😉

I changed the test_build_inputs_with_special_tokens to test this edge case :)

Thanks 😉 

> Very good thanks, I would just like 1 hard coded expected value, otherwise if both are broken we are not testing anything! Thanks for the fix

Can you give me some guidance on how to do this?
Because the behaviour differs from tokenizer to tokenizer, I can't/shouldn't do it in test_tokenization_common, right?
So instead I hard code one test in each test_tokenization_(tokenizer)?

Oups my bad on this one! 
It's alright like this 😉 I'll merge