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
857301324,MDU6SXNzdWU4NTczMDEzMjQ=,5151,"DataArray.mean() emits warning with Dask, not NumPy",1312546,closed,0,,,3,2021-04-13T20:34:56Z,2021-09-15T16:41:43Z,2021-09-15T16:41:43Z,MEMBER,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

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**What happened**:

When calling DataArray.mean on an all-NaN dataset, a warning is emitted if and only if a Dask array is used.

**What you expected to happen**:

Identical behavior between the two, probably no warning .

**Minimal Complete Verifiable Example**:

```python
In [7]: import xarray as xr

In [8]: import numpy as np

In [9]: import dask.array as da

In [10]: import xarray as xr

In [11]: a = xr.DataArray(da.from_array(np.full((10, 10), np.nan)))

In [12]: a.mean(dim=""dim_0"").compute()
/home/taugspurger/miniconda3/envs/tmp-adlfs/lib/python3.8/site-packages/dask/array/numpy_compat.py:39: RuntimeWarning: invalid value encountered in true_divide
  x = np.divide(x1, x2, out)
Out[12]:
<xarray.DataArray 'array-395d894c4e4d4ca165a189736da1f52d' (dim_1: 10)>
array([nan, nan, nan, nan, nan, nan, nan, nan, nan, nan])
Dimensions without coordinates: dim_1

In [13]: a.compute().mean(dim=""dim_0"")
Out[13]:
<xarray.DataArray 'array-395d894c4e4d4ca165a189736da1f52d' (dim_1: 10)>
array([nan, nan, nan, nan, nan, nan, nan, nan, nan, nan])
Dimensions without coordinates: dim_1
```

**Anything else we need to know?**:

I haven't looked closely at why this is happening (I couldn't immediately find where `.mean` is reduced). I know that Dask has had some issues in the past where NumPy warnings filters are set during *graph construction* time, but aren't set when the graph is actually computed.

**Environment**:

<details>

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.8.8 | packaged by conda-forge | (default, Feb 20 2021, 16:22:27)
[GCC 9.3.0]
python-bits: 64
OS: Linux
OS-release: 5.4.72-microsoft-standard-WSL2
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: C.UTF-8
LOCALE: en_US.UTF-8
libhdf5: None
libnetcdf: None

xarray: 0.17.0
pandas: 1.2.4
numpy: 1.20.2
scipy: None
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: 2.7.0
cftime: None
nc_time_axis: None
PseudoNetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: None
dask: 2021.04.0
distributed: 2021.04.0
matplotlib: None
cartopy: None
seaborn: None
numbagg: None
pint: None
setuptools: 52.0.0.post20210125
pip: 21.0.1
conda: None
pytest: None
IPython: 7.22.0
sphinx: None
```

</details>
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