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
494210818,MDExOlB1bGxSZXF1ZXN0MzE4MDA5MjU3,3312,convert DataArray to DataSet before combine,10554254,open,0,,,10,2019-09-16T18:37:35Z,2022-06-09T14:50:17Z,,FIRST_TIME_CONTRIBUTOR,,0,pydata/xarray/pulls/3312,"Enables combine_by_coords on DataArrays. Will convert DataArray to DataSet before proceeding. 

As mentioned in #3248, this will still fail if the DataArray is unnamed, but at least the error message tells the user why.

Previously, combining both named 

```
da1 = xr.DataArray(name='foo', 
                  data=np.random.rand(3,3), 
                  coords=[('x', [1, 2, 3]),
                          ('y', [1, 2, 3])])

da2 = xr.DataArray(name='foo2', 
                  data=np.random.rand(3,3), 
                  coords=[('x', [5, 6, 7]),
                          ('y', [5, 6, 7])])

xr.combine_by_coords([da1, da2])
```

and unnamed DataArrays
```
da1 = xr.DataArray(data=np.random.rand(3,3), 
                  coords=[('x', [1, 2, 3]),
                          ('y', [1, 2, 3])])

da2 = xr.DataArray(data=np.random.rand(3,3), 
                  coords=[('x', [5, 6, 7]),
                          ('y', [5, 6, 7])])

xr.combine_by_coords([da1, da2])
```
failed with `ValueError: The truth value of an array with more than one element is ambiguous. Use a.any() or a.all()`

With this PR, combining the named DataArrays results in a combined DataSet, while the latter example will result in
`ValueError: unable to convert unnamed DataArray to a Dataset without providing an explicit name`","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3312/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,pull