Bert: Sometime I get running error, maybe it is a bug

[lixinsu]

https://github.com/google-research/bert/blob/4a47cc2da23dcb4ab1bf5c08085910b6fd94a4cf/run_squad.py#L865-L890
This may cause best_non_null_entry is None when FLAGS.version_2_with_negative is True, and I think if not nbest should be moved up.

[ymcui]

Yes, sometimes the best_non_null_entry can be null and breaks the prediction process. It would be better to add a if/else to avoid such unexpected behavior.