Indexing with an empty sequence results in an exception

[mdavezac]

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

# 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()

# Paste the output here xr.show_versions() here commit: 2c10d14 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