Fixing QA when `sequence_ids` is None (instead of 0, 1).
Author: Narsil
Pull request: yes

# What does this PR do?

Attempts to fix flaky test (and bug).

`tokenizer.sequence_ids(span)` was returning [None, 0, 0, 0, None, 1, 1, 1,1 , None]
and the test to make the mask for answers in question-answering is

[tok != 1 if question_first else 0].

1 and 0 are both != None so all special tokens would be included in 
potential answers. With tests on random models it could happen triggereing
a bug where offsets would be out of bounds since the last None is out of bounds
for the offsets of the question for instance.

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Fixes # (issue)


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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## Who can review?

@stas00
@sgugger


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Comments:
This is incorrect actually