Fix to prevent crashing on assert len(tokens_b)>=1
Thank you for the awesome library! One little issue that I have sometimes with somewhat "noisy" text is that Bert tokenizer fails to process some weird stuff. In such a case, one unexpectedly gets no tokens and the converter fails on assert with a message like this one:
```
Epoch:   0%|          | 0/1 [00:00<?, ?it/s]                      Traceback (most recent call last):
  File "../pytorch-transformers/examples/lm_finetuning/pregenerate_training_data.py", line 354, in <module>
    main()
  File "../pytorch-transformers/examples/lm_finetuning/pregenerate_training_data.py", line 350, in main
    create_training_file(docs, vocab_list, args, epoch)
  File "../pytorch-transformers/examples/lm_finetuning/pregenerate_training_data.py", line 276, in create_training_file
    whole_word_mask=args.do_whole_word_mask, vocab_list=vocab_list)
  File "../pytorch-transformers/examples/lm_finetuning/pregenerate_training_data.py", line 244, in create_instances_from_document   
    assert len(tokens_b) >= 1
AssertionError

```
Ok, merging.
Note that we are in the process of deprecating these finetuning scripts and replacing them with the common `run_lm_finetuning.py` which handles several models.<|||||>Thank you @thomwolf sorry didn't realize this was deprecated.