Remove special mapping of `auto` to {} in `open_zarr`

[jsignell]

• [x] Closes #11002
• [x] Tests added
• [x] User visible changes (including notable bug fixes) are documented in whats-new.rst

This PR makes the handling of chunks="auto" consistent between open_zarr and open_dataset(..., engine="zarr").

The handling of chunks still differs in open_zarr vs open_dataset(..., engine="zarr") in that the default in open_zarr is to use chunks="auto" and a chunk manager (aka dask) when available in your env. And in open_dataset the default is to use chunks=None (aka no chunks).