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
431412346,MDU6SXNzdWU0MzE0MTIzNDY=,2882,Indexing with an empty sequence results in an exception,2745737,closed,0,,,0,2019-04-10T09:58:24Z,2019-04-12T00:25:44Z,2019-04-12T00:25:44Z,CONTRIBUTOR,,,,"#### Code Sample, a copy-pastable example if possible

A ""Minimal, Complete and Verifiable Example"" will make it much easier for maintainers to help you:
http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports

```python
# Your code here
da = DataArray(np.arange(12).reshape((3, 4)), dims=['x', 'y'])
da[{'x': [0]}]  # Okay
da[{'x': []}] # error 
```
#### Problem description

Indexing with an empty list results in an exception.

#### Expected Output

It should return a dataarray with an axis of length zero.

#### Output of ``xr.show_versions()``

<details>
# Paste the output here xr.show_versions() here
commit: 2c10d1443bea09e5ef53e5a7e35195a195e193a7
python: 3.7.3 (default, Mar 27 2019, 09:23:15)
[Clang 10.0.1 (clang-1001.0.46.3)]
python-bits: 64
OS: Darwin
OS-release: 18.5.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_GB.UTF-8
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8
libhdf5: None
libnetcdf: None

xarray: 0.12.1+4.g2c10d144.dirty
pandas: 0.24.2
numpy: 1.16.2
scipy: 1.2.1
netCDF4: None
pydap: None
h5netcdf: None
h5py: None
Nio: None
zarr: None
cftime: None
nc_time_axis: None
PseudonetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.2.1
dask: None
distributed: None
matplotlib: 3.0.3
cartopy: None
seaborn: None
setuptools: 41.0.0
pip: 19.0.3
conda: None
pytest: 4.3.1
IPython: 7.4.0
sphinx: None
</details>
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