Fix "No such file or directory" for SQuAD v1.1
This solves the exception for SQuAD v1.1 evaluation without predicted null_odds file.

Traceback (most recent call last):
  File "./examples/run_squad.py", line 521, in <module>
  File "./examples/run_squad.py", line 510, in main
    for checkpoint in checkpoints:
  File "./examples/run_squad.py", line 257, in evaluate
    na_prob_file=output_null_log_odds_file)
  File "/home/zhangzs/pytorch-transformers-master/examples/utils_squad_evaluate.py", line 291, in main
    with open(OPTS.na_prob_file) as f:
FileNotFoundError: [Errno 2] No such file or directory: 'squad/squad-debug/null_odds_.json'
# [Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1079?src=pr&el=h1) Report
> Merging [#1079](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1079?src=pr&el=desc) into [master](https://codecov.io/gh/huggingface/pytorch-transformers/commit/e00b4ff1de0591d5093407b16e665e5c86028f04?src=pr&el=desc) will **decrease** coverage by `0.02%`.
> The diff coverage is `n/a`.

[![Impacted file tree graph](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1079/graphs/tree.svg?width=650&token=9qOlN6Hb1c&height=150&src=pr)](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1079?src=pr&el=tree)

```diff
@@            Coverage Diff             @@
##           master    #1079      +/-   ##
==========================================
- Coverage   79.61%   79.58%   -0.03%     
==========================================
  Files          42       42              
  Lines        6898     6898              
==========================================
- Hits         5492     5490       -2     
- Misses       1406     1408       +2
```


| [Impacted Files](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1079?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pytorch\_transformers/tokenization\_bert.py](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1079/diff?src=pr&el=tree#diff-cHl0b3JjaF90cmFuc2Zvcm1lcnMvdG9rZW5pemF0aW9uX2JlcnQucHk=) | `93.89% <0%> (-0.94%)` | :arrow_down: |

------

[Continue to review full report at Codecov](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1079?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/pytorch-transformers/pull/1079?src=pr&el=footer). Last update [e00b4ff...61f14c5](https://codecov.io/gh/huggingface/pytorch-transformers/pull/1079?src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
<|||||>I think I fixed the issue about month ago?
https://github.com/huggingface/pytorch-transformers/blob/e00b4ff1de0591d5093407b16e665e5c86028f04/examples/run_squad.py#L248-L251<|||||>Thx! It looks fine now. My version is out of date. I'll close the comment.