Fixes NoneType exception when topk is larger than one coupled with a small context in the Question-Answering pipeline
Author: psorianom
Pull request: yes

# What does this PR do?

This PR fixes a bug while using the QA pipeline where if `topk` > 1 (e.g., 20) and the context of a Question-Context pair is short, the pipeline will propose non-context words as candidates for the answer span. This will generate a `NoneType` error down the line.
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Fixes  https://github.com/huggingface/transformers/issues/11354


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Comments:
Great, thanks! Could you fix the quality issues in your code? You can do so with the following:

```
pip install -e .[quality]
make fixup
```

> Great, thanks! Could you fix the quality issues in your code? You can do so with the following:
> 
> ```
> pip install -e .[quality]
> make fixup
> ```

yes ! thanks for the command. It is done :)