considering empty input text to avoid string index out of range error
When the input string is empty, the above condition in the tokenization_roberta will encounter string index out of range error. For example, a pair input string exists in the QQP dataset that has an empty string:

{'idx': 362246,
 'label': 0,
 'question1': b'How can I develop android app?',
 'question2': b''}
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