issues: 572995385

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id node_id number title user state locked assignee milestone comments created_at updated_at closed_at author_association active_lock_reason draft pull_request body reactions performed_via_github_app state_reason repo type
572995385 MDU6SXNzdWU1NzI5OTUzODU= 3811 Don't warn on empty reductions 5635139 closed 0     2 2020-02-28T20:45:38Z 2021-02-21T23:05:46Z 2021-02-21T23:05:46Z MEMBER      

Numpy warns when computing over an all-NaN slice. We handle that case reasonably and so should handle and discard the warning.

MCVE Code Sample

```python In [1]: import xarray as xr

In [2]: import numpy as np

In [3]: da = xr.DataArray(np.asarray([np.nan]*3))

In [4]: da
Out[4]: <xarray.DataArray (dim_0: 3)> array([nan, nan, nan]) Dimensions without coordinates: dim_0

In [6]: da.mean()
[...]/python3.6/site-packages/xarray/core/nanops.py:142: RuntimeWarning: Mean of empty slice return np.nanmean(a, axis=axis, dtype=dtype) Out[6]: <xarray.DataArray ()> array(nan)

```

Expected Output

No warning

Problem Description

Somewhat discussed in https://github.com/pydata/xarray/issues/1164, and https://github.com/pydata/xarray/issues/1652, but starting a separate issue as it's more important than just noise in the test suite, and not covered by the existing work on comparisons & arithmetic

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   completed 13221727 issue

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