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
668905666,MDU6SXNzdWU2Njg5MDU2NjY=,4291,resample function gives 0s instead of NaNs,8161792,closed,0,,,3,2020-07-30T15:59:32Z,2020-08-05T16:55:58Z,2020-08-05T16:55:58Z,NONE,,,,"<!-- Please include a self-contained copy-pastable example that generates the issue if possible.

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**What happened**:
When I use `resample(time='1d').sum(dim='time')` to resample a time series with NaNs, the resampled result gives me 0s instead of NaNs, while NaNs should be the correct answer.

**What you expected to happen**:

NaNs should be the correct answer.

**Minimal Complete Verifiable Example**:

```python
import xarray as xr

dates =  pd.date_range('20200101', '20200601', freq='h')
data = np.linspace(0, 10, num=len(dates))
data[0:30*24] = np.nan

da = xr.DataArray(data, coords=[dates], dims='time')
da.plot()

# Instead of NaNs, the resampled time series in January 20202 give us 0s, which not right.
da.resample(time='1d', skipna=True).sum(dim='time', skipna=True).plot()
```

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

Did I misunderstand something here? Thanks!


**Environment**:
xarray - '0.15.1' 

<details><summary>Output of <tt>xr.show_versions()</tt></summary>

xarray - '0.15.1' 


</details>
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