Empty output/answer in TriviaQA test set (both in 'kilt_tasks' and 'trivia_qa')
# The issue

It's all in the title, it appears to be fine on the train and validation sets.

Is there some kind of mapping to do like for the questions (see https://github.com/huggingface/datasets/blob/master/datasets/kilt_tasks/README.md) ? 

# How to reproduce
```py
from datasets import load_dataset
kilt_tasks = load_dataset("kilt_tasks")
trivia_qa = load_dataset('trivia_qa', 'unfiltered.nocontext')
# both in "kilt_tasks"
In [18]: any([output['answer'] for output in kilt_tasks['test_triviaqa']['output']])                                                                                                                        
Out[18]: False
# and "trivia_qa"
In [13]: all([answer['value'] == '<unk>' for answer in trivia_qa['test']['answer']])                                                                                                                        
Out[13]: True
# appears to be fine on the train and validation sets.
In [14]: all([answer['value'] == '<unk>' for answer in trivia_qa['train']['answer']])                                                                                                                       
Out[14]: False

In [15]: all([answer['value'] == '<unk>' for answer in trivia_qa['validation']['answer']])                                                                                                                  
Out[15]: False

In [16]: any([output['answer'] for output in kilt_tasks['train_triviaqa']['output']])                                                                                                                       
Out[16]: True

In [17]: any([output['answer'] for output in kilt_tasks['validation_triviaqa']['output']])                                                                                                                  
Out[17]: True

```