missing empty group when iterate over groupby_bins

When I try to iterate over the object grouped returned by groupby_bins, I found that the empty group is missing silently. Here is a simple case:

array = xr.DataArray(np.arange(4), dims='dim_0')
# one of these bins will be empty
bins = [0,4,5]
grouped = array.groupby_bins('dim_0', bins)

for i, group in enumerate(grouped):
    print(str(i)+'  '+group)

When a bin contains no samples (bin of (4, 5]), the empty group will be dropped. Then how to iterate over the full bins even when some bins contain nothing? I've read this related issue #1019.
But my case here need the correct order in grouped and empty groups need to be iterated over.

Run the provided Python reproduction and inspect the groupby_bins iteration entry point in xarray. Confirm how bins are represented when one has no samples; done means iteration preserves bin order and yields the empty bin rather than silently dropping it.