Provide documentation on what is considered missing units

In ClimaAnalysis, if the attributes does not have "units" as a key or if the attributes does, but the value is the empty string, then the units returned by `units` is the empty string.

The documentation for `units` implies that the units is missing. This seems to be the behavior in the other ClimaAnalysis functions.

It would be nice to say this somewhere in the documentation.

Furthermore, this behavior differs from how `UDUnits.jl` parse units as the empty string is considered unitless and one can argue that if the units attribute is present, then the creators of the dataset intended for the units to be unitless. It may be worth it to update this behavior, but this would be considered a breaking change.

Find the documentation for the ClimaAnalysis `units` function and read its current description and nearby examples. Document that missing `units` attributes and empty-string values return an empty string, and clearly separate that behavior from the possible breaking change involving UDUnits.jl parsing.

Written by the indexing model from the issue text.