Discard empty corpus before computation

[devrimcavusoglu]

Currently given corpus like below causes problems as they are not expecting an empty list:

import jury

p = [["a b c"], []]
r = [["a b d e f"], ["a g h i"]]

scorer = jury.Jury()
scores = scorer(predictions=p, references=r)

the code above throws an exception when encounters an empty list:

Traceback (most recent call last):
  File "/home/devrimcavusoglu/lab/gh/jury/jury/core.py", line 202, in <module>
    scores = scorer(predictions=p, references=r)
  File "/home/devrimcavusoglu/lab/gh/jury/jury/core.py", line 79, in __call__
    score = self._compute_single_score(inputs)
  File "/home/devrimcavusoglu/lab/gh/jury/jury/core.py", line 148, in _compute_single_score
    score = metric.compute(predictions=predictions, references=references, reduce_fn=reduce_fn)
  File "/home/devrimcavusoglu/lab/gh/jury/venv/lib/python3.8/site-packages/datasets/metric.py", line 402, in compute
    output = self._compute(predictions=predictions, references=references, **kwargs)
  File "/home/devrimcavusoglu/lab/gh/jury/jury/metrics/_core/base.py", line 325, in _compute
    result = self.evaluate(predictions=predictions, references=references, reduce_fn=reduce_fn, **eval_params)
  File "/home/devrimcavusoglu/lab/gh/jury/jury/metrics/bleu/bleu_for_language_generation.py", line 262, in evaluate
    return super().evaluate(predictions=predictions, references=references, reduce_fn=reduce_fn, **kwargs)
  File "/home/devrimcavusoglu/lab/gh/jury/jury/metrics/_core/base.py", line 279, in evaluate
    return eval_fn(predictions=predictions, references=references, **kwargs)
  File "/home/devrimcavusoglu/lab/gh/jury/jury/metrics/bleu/bleu_for_language_generation.py", line 216, in _compute_multi_pred_multi_ref
    adjusted_prediction_length += get_token_lengths(preds, reduce_fn=max)
  File "/home/devrimcavusoglu/lab/gh/jury/jury/metrics/_core/utils.py", line 58, in get_token_lengths
    return int(reduce_fn(token_lengths))
ValueError: max() arg is an empty sequence

Process finished with exit code 1