id,node_id,number,state,locked,title,user,body,created_at,updated_at,closed_at,merged_at,merge_commit_sha,assignee,milestone,draft,head,base,author_association,auto_merge,repo,url,merged_by
318009257,MDExOlB1bGxSZXF1ZXN0MzE4MDA5MjU3,3312,open,0,convert DataArray to DataSet before combine,10554254,"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`",2019-09-16T18:37:35Z,2022-06-09T14:50:17Z,,,e7a2e3b14bb20872a03f0a9e96339db46512a0c9,,,0,53afe58ae12f1b0e5873638b7a05db0845862b8c,d1e4164f3961d7bbb3eb79037e96cae14f7182f8,FIRST_TIME_CONTRIBUTOR,,13221727,https://github.com/pydata/xarray/pull/3312,