Ignore empty batch when writing
This prevents an schema update with unknown column types, as reported in #2644.

This is my first attempt at fixing the issue. I tested the following:
- First batch returned by a batched map operation is empty.
- An intermediate batch is empty.
- `python -m unittest tests.test_arrow_writer` passes.

However, `arrow_writer` looks like a pretty generic interface, I'm not sure if there are other uses I may have overlooked. Let me know if that's the case, or if a better approach would be preferable.