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
285349783,MDU6SXNzdWUyODUzNDk3ODM=,1807,Empty DataArray should have a length of 0,13813076,closed,0,,,4,2018-01-02T03:24:08Z,2019-01-22T22:49:27Z,2019-01-22T22:49:27Z,NONE,,,,"
```python
dr = xr.DataArray(())
len(dr)
```

When evaluating the length of an empty DataArray, instead of evaluating as zero, it raises a TypeError. Most other container classes (lists, dicts, ndarrays) all evaluate with a zero length when empty. I would expect and prefer that this class evaluate its length as zero when empty.

<details>
xr.show_versions()

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.1.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

xarray: 0.10.0
pandas: 0.21.1
numpy: 1.13.3
scipy: 1.0.0
netCDF4: None
h5netcdf: None
Nio: None
bottleneck: None
cyordereddict: None
dask: None
matplotlib: None
cartopy: None
seaborn: None
setuptools: 38.2.5
pip: 9.0.1
conda: None
pytest: None
IPython: 6.2.1
sphinx: None

</details>
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