I think this warning code is a bug and everything should consistently use 'empty-file' instead. The only way to make 'emptyfile' happen is to somehow get a zero-length file into upload stash (which is impossible, as both frontends [special page and API module] and the upload backend actively reject zero-length files), so this is realistically not going to be a breaking change, but technically it does change the API output. A legit 'empty-file' is returned from the API as an error (not a warning) when trying to upload an empty file, and potentially also as a warning but I couldn't find a way to trigger this.