Better error message when one forgets to call `add_batch` before `compute`
When using metrics, if for some reason a user forgets to call `add_batch` to a metric before `compute` (with no arguments), the error message is a bit cryptic and could probably be made clearer.

## Reproducer

```python
import datasets
import torch
from datasets import Metric

class GatherMetric(Metric):
    def _info(self):
        return datasets.MetricInfo(
            description="description",
            citation="citation",
            inputs_description="kwargs",
            features=datasets.Features({
                'predictions': datasets.Value('int64'),
                'references': datasets.Value('int64'),
            }),
            codebase_urls=[],
            reference_urls=[],
            format='numpy'
        )

    def _compute(self, predictions, references):
        return {"predictions": predictions, "labels": references}

metric = GatherMetric(cache_dir="test-metric")
inputs = torch.randint(0, 2, (1024,))
targets = torch.randint(0, 2, (1024,))

batch_size = 8
for i in range(0, 1024, batch_size):
     pass # User forgets to call `add_batch`
result = metric.compute()
```

## Stack trace:

```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-13-267729d187fa> in <module>
      3      pass
      4     # metric.add_batch(predictions=inputs[i:i+batch_size], references=targets[i:i+batch_size])
----> 5 result = metric.compute()

~/git/datasets/src/datasets/metric.py in compute(self, *args, **kwargs)
    380         if predictions is not None:
    381             self.add_batch(predictions=predictions, references=references)
--> 382         self._finalize()
    383 
    384         self.cache_file_name = None

~/git/datasets/src/datasets/metric.py in _finalize(self)
    343         elif self.process_id == 0:
    344             # Let's acquire a lock on each node files to be sure they are finished writing
--> 345             file_paths, filelocks = self._get_all_cache_files()
    346 
    347             # Read the predictions and references

~/git/datasets/src/datasets/metric.py in _get_all_cache_files(self)
    280         filelocks = []
    281         for process_id, file_path in enumerate(file_paths):
--> 282             filelock = FileLock(file_path + ".lock")
    283             try:
    284                 filelock.acquire(timeout=self.timeout)

TypeError: unsupported operand type(s) for +: 'NoneType' and 'str'
```
