Don't write unnecessary _FillValue=NaN for Zarr files

[shoyer]

What is your issue?

As pointed out by @rabernat in #10757, there is no point in writing _FillValue attributes set to NaN in Zarr files.

This makes Zarr stores created with Xarray a bit more complex than necessary, and may unnecessarily invoke Xarray's masking / fill value machinery.

[rabernat]

@DahnJ would this interfere with how you are currently using Zarr? Do you have code which expects the _FillValue attribute to be present?