[Feature Request] Check the length of batch before writing so that empty batch is allowed
I think this error means you filter all examples within an (input) batch by deleting its columns. In that case, to avoid the error, you can set the column value to an empty list (`input_batch["col"] = []`) instead.
### 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
