Add support for the null answer in `QuestionAnsweringPipeline`
# [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3441?src=pr&el=h1) Report
> Merging [#3441](https://codecov.io/gh/huggingface/transformers/pull/3441?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/transformers/commit/010e0460b22ddd7f74e31163f69ab3da2e9741ba&el=desc) will **decrease** coverage by `0.00%`.
> The diff coverage is `66.66%`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/transformers/pull/3441/graphs/tree.svg?width=650&height=150&src=pr&token=9qOlN6Hb1c)](https://codecov.io/gh/huggingface/transformers/pull/3441?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #3441      +/-   ##
==========================================
- Coverage   77.61%   77.60%   -0.01%     
==========================================
  Files         100      100              
  Lines       16972    16978       +6     
==========================================
+ Hits        13172    13175       +3     
- Misses       3800     3803       +3     
```


| [Impacted Files](https://codecov.io/gh/huggingface/transformers/pull/3441?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [src/transformers/pipelines.py](https://codecov.io/gh/huggingface/transformers/pull/3441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9waXBlbGluZXMucHk=) | `72.44% <66.66%> (-0.09%)` | :arrow_down: |
| [src/transformers/modeling\_tf\_utils.py](https://codecov.io/gh/huggingface/transformers/pull/3441/diff?src=pr&el=tree#diff-c3JjL3RyYW5zZm9ybWVycy9tb2RlbGluZ190Zl91dGlscy5weQ==) | `83.94% <0.00%> (-0.18%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/transformers/pull/3441?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/huggingface/transformers/pull/3441?src=pr&el=footer). Last update [010e046...8671bc3](https://codecov.io/gh/huggingface/transformers/pull/3441?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>Hi @bryant1410, thanks for pushing this into the QA pipeline.

My only concern is about the name of the parameter `version_2_with_negative` introduced in this PR. It seems very tight up to SQuAD2 and might be hard for newcomers to directly understand what it does.

Would you mind changing the name of the parameter `version_2_with_negative` to `handle_impossible_answer` ? <|||||>Sure. Btw, is it okay for it to default to `False`?<|||||>LGTM too, thanks @bryant1410 