e.g., when using taxize, a user will often have some taxa with matches and some with no matches. We then have to turn the results into taxa pkg classes. Right now taxa classes can't do no data unless pass in e.g,. empty string like "" -

maybe we should explicitly handle this so that "null" classes are supported and are supported when combining classes, etc. (supported could mean for null objects that they are dropped or something, not sure yet)