Question answeriung model, unanswerable question answer formatformat

Hello, I am working with question answering model right now and I am new to this kind of task.

If I have a question that can not be answered using the provided context, what is the format of answer?
For examples in squad dataset

question: To whom did the Virgin Mary allegedly appear in 1858 in Lourdes France?
answer: { "text": [ "Saint Bernadette Soubirous" ], "answer_start": [ 515 ] }

but what if this question can not be answer, how should I prepare answer: … ?
Should it be something like answer: { "text": [ ], "answer_start": [ -1 ] } ??