Change document question answering pipeline to always return an array
# What does this PR do?

Updates the DocumentQuestionAnsweringPipeline to always return an array, to fix the inference widget, and also be easier to use in general.

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x ] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
- x[ ] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [x ] Did you write any new necessary tests?


## Who can review?

@Narsil @mishig25 


_The documentation is not available anymore as the PR was closed or merged._<|||||>i think this will fix this issue:

<img width="686" alt="image" src="https://user-images.githubusercontent.com/326577/191121642-1fd004ea-4111-439b-923c-020acf05c5b7.png">
<|||||>Yes that is exactly its intent!