Fix some bug about SQuAD code
Fix issue in #207 
![image](https://user-images.githubusercontent.com/16603773/52842280-90939a00-3139-11e9-87e8-92bcfb8af76d.png)

This error occurs when 'nbest' only contain 1 item, but none 'text'. So the code to add empty will not work.
I add another condition to solve it.

https://github.com/huggingface/pytorch-pretrained-BERT/blob/master/examples/run_squad.py#L570-L590
Ok, thanks @wlhgtc!