[Feature Request] Check the length of batch before writing so that empty batch is allowed
### Use Case
I use `dataset.map(process_fn, batched=True)` to process the dataset, with data **augmentations or filtering**. However, when all examples within a batch is filtered out, i.e. **an empty batch is returned**, the following error will be thrown:
  ```
  ValueError: Schema and number of arrays unequal
  ```
This is because the empty batch does not comply with the schema of other batches. I think an empty batch should be allowed to facilitate coding (one does not need to assign an empty list manually for all keys.)

A simple fix is to check the length of `batch` before writing:
```
if len(batch):
  writer.write_batch(batch)
```
instead of 

https://github.com/huggingface/datasets/blob/74d60213dcbd7c99484c62ce1d3dfd90a1df0770/src/datasets/arrow_dataset.py#L3493
