groupby on empty DataArray raises StopIteration #3037
Description
This seems similar to #1764 and #2240 so apologies if it is a duplicate, but I have a minimal example where it happens on an empty DataArray:
Code Sample
import xarray as xr
xr.DataArray([], dims='dim').groupby('dim').mean() # raises StopIterationProblem Description
Using groupby on an empty DataArray or Dataset raises StopIteration. It should raise a more meaningful error.
In particular, I had this issue in a function I was calling inside of a generator, so the StopIteration just broke out of the generator and it took some digging to figure out what was going wrong in my code.
Output of xr.show_versions()
Details
INSTALLED VERSIONS
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 3.10.0-862.14.4.el7.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.utf8
LOCALE: en_US.UTF-8
xarray: 0.10.7
pandas: 0.23.0
numpy: 1.14.3
scipy: 1.1.0
netCDF4: 1.4.0
h5netcdf: 0.6.1
h5py: 2.7.1
Nio: None
zarr: None
bottleneck: 1.2.1
cyordereddict: None
dask: 0.17.5
distributed: 1.21.8
matplotlib: 2.2.2
cartopy: None
seaborn: 0.8.1
setuptools: 39.1.0
pip: 10.0.1
conda: None
pytest: 3.5.1
IPython: 6.4.0
sphinx: 1.7.4