Issue: KILT dataset: empty string in triviaqa input field

# What happened
Both train and test splits of the triviaqa dataset (part of the KILT benchmark) seem to have empty string in their input field (unlike the natural questions dataset, part of the same benchmark)

# Versions
KILT version is `1.0.0`
`datasets` version is `1.1.2`
[more here](https://gist.github.com/PaulLerner/3768c8d25f723edbac20d99b6a4056c1)

# How to reproduce
```py
In [1]: from datasets import load_dataset
In [4]: dataset = load_dataset("kilt_tasks")                                                                                                                                                                
# everything works fine, removed output for a better readibility
Dataset kilt_tasks downloaded and prepared to /people/lerner/.cache/huggingface/datasets/kilt_tasks/all_tasks/1.0.0/821c4295a2c35db2847585918d9c47d7f028f1a26b78825d8e77cd3aeb2621a1. Subsequent calls will reuse this data.

# empty string in triviaqa input field
In [36]: dataset['train_triviaqa'][0]                                                                                                                                                                       
Out[36]: 
{'id': 'dpql_5197',
 'input': '',
 'meta': {'left_context': '',
  'mention': '',
  'obj_surface': {'text': []},
  'partial_evidence': {'end_paragraph_id': [],
   'meta': [],
   'section': [],
   'start_paragraph_id': [],
   'title': [],
   'wikipedia_id': []},
  'right_context': '',
  'sub_surface': {'text': []},
  'subj_aliases': {'text': []},
  'template_questions': {'text': []}},
 'output': {'answer': ['five  £', '5 £', '£5', 'five £'],
  'meta': [],
  'provenance': [{'bleu_score': [1.0],
    'end_character': [248],
    'end_paragraph_id': [30],
    'meta': [],
    'section': ['Section::::Question of legal tender.\n'],
    'start_character': [246],
    'start_paragraph_id': [30],
    'title': ['Banknotes of the pound sterling'],
    'wikipedia_id': ['270680']}]}}
In [35]: dataset['train_triviaqa']['input'][:10]                                                                                                                                                            
Out[35]: ['', '', '', '', '', '', '', '', '', '']
# same with test set 
In [37]: dataset['test_triviaqa']['input'][:10]                                                                                                                                                             
Out[37]: ['', '', '', '', '', '', '', '', '', '']
# works fine with natural questions
In [34]: dataset['train_nq']['input'][:10]                                                                                                                                                                  
Out[34]: 
['how i.met your mother who is the mother',
 'who had the most wins in the nfl',
 'who played mantis guardians of the galaxy 2',
 'what channel is the premier league on in france',
 "god's not dead a light in the darkness release date",
 'who is the current president of un general assembly',
 'when do the eclipse supposed to take place',
 'what is the name of the sea surrounding dubai',
 'who holds the nba record for most points in a career',
 'when did the new maze runner movie come out']
```

Stay safe :)

Comments:
Just found out about https://github.com/huggingface/datasets/blob/master/datasets/kilt_tasks/README.md
(Not very clear in https://huggingface.co/datasets/kilt_tasks links to http://github.com/huggingface/datasets/datasets/kilt_tasks/README.md which is dead, closing the issue though :))