The script seems to generally assume that "" is a good representation for "no prefix", i.e. the default namespace, although that is IMHO more correctly represented as None. It's not very likely that this is the only script out there that makes that assumption.

In fact, this might not be an entirely stupid assumption, given that None doesn't sort together with strings, for example. And sorting prefixes is not an unusual thing to do.

That makes it a case of "practicality beats purity", I guess …

I'll change the implementation to allow empty strings.