`.fillna()` slower than expected for sparse data arrays with `fill_value=nan`

[staadecker]

What is your issue?

Expected behavior

.fillna(0) should be near instantaneous when applied to a sparse DataArray with fill_value=nan

Why

.fillna(0) only needs to update the fill_value to 0.

Current behaviour

The normal .where() operation is applied on the DataArray instead of using the shortcut described above.

Question

What would be required to improve the performance of fillna()? I'm happy to try taking a stab at it if pointed in the right direction.

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