Quote and single quote are not handled correctly in vocab file where words are not wrapped in quotes

[hepaajan]

Especially following branch will remove the quote so that it becomes empty string (as single quote character starts and ends with quote):

https://github.com/tensorflow/tensor2tensor/blob/5f9dd2db6d7797162e53adf152310ed13e9fc711/tensor2tensor/data_generators/text_encoder.py#L929

easy fix is the check also that "len(s) > 1" in both conditions