Singleton coordinates crash the core dim order check

[aulemahal]

In the core dim order check added by #582, we forgot the case where a coordinate would have only one element. When this happens, the diff is an empty array and the function fails here:

cf-xarray/cf_xarray/helpers.py

Line 234 in e6b7915

elif isinstance(diffs[0], datetime.timedelta):

The easy solution would be to consider them ascending, as we already do with coordinates that are all the same.