`write_empty_chunks` not in `DataArray.to_zarr`

[max-sixty]

What is your issue?

Our to_zarr methods on DataArray & Dataset are slightly inconsistent — Dataset.to_zarr has write_empty_chunks and chunkmanager_store_kwargs. They're also in a different order.

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Up a level — not sure of the best way of enforcing consistency here; a couple of ideas.

• We could have tests that operate on both a DataArray and Dataset, parameterized by fixtures (might also help reduce the duplication in some of our tests), though we then need to make the tests generic. We could have some general tests which just test that methods work, and then delegate to the current per-object tests for finer guarantees.
• We could have a tool which collects the differences between DataArray & Dataset methods and snapshots them — then we'll see if they diverge, while allowing for some divergences.