Fix initializing BertJapaneseTokenizer with AutoTokenizers
# What does this PR do?

This PR fixes a bug in loading some kinds of tokenizers using `AutoTokenizers.from_pretrained()`.
This issue is discussed in https://github.com/cl-tohoku/bert-japanese/issues/25.

When `sentencepiece` is not installed, the initialization of several tokenizers such as `BertJapaneseTokenizer`, `BarthezTokenizer`, and `MBart50Tokenizer` fails.
The exception is raised in `tokenizer_class_from_name()` when iterating over tokenizer classes which are `NoneType` objects. 
Such tokenizer classes are set to `None` in `is_sentencepiece_available()` if `sentencepiece` is not available.

This error affects the initialization of `BertJapaneseTokenizer` even though it does not depend on `sentencepiece`.

```sh
>>> from transformers import AutoTokenizer
>>> tokenizer = AutoTokenizer.from_pretrained("cl-tohoku/bert-base-japanese")
Downloading: 100%|███████████████████████████████████████████████████████████████████████████████████████████████████████████████| 479/479 [00:00<00:00, 143kB/s]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/m-suzuki/.pyenv/versions/py3.7/lib/python3.7/site-packages/transformers/models/auto/tokenization_auto.py", line 370, in from_pretrained
    tokenizer_class = tokenizer_class_from_name(tokenizer_class_candidate)
  File "/Users/m-suzuki/.pyenv/versions/py3.7/lib/python3.7/site-packages/transformers/models/auto/tokenization_auto.py", line 273, in tokenizer_class_from_name
    if c.__name__ == class_name:
AttributeError: 'NoneType' object has no attribute '__name__'
```

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## Who can review?

@LysandreJik
