How do you generate the sitemap? Ditto, Wayfinder, manually? I don't think it's really empty string that's causing the error, but some entity that tinyMCE didn't recognize. But either way: Try to erase / delete any non-needed line-breaks and empty spaces in your Ditto templates and / or the document / template where the snippet call is placed. Perhaps you copy-and-pasted code from a manual / tutorial? If I download the sitemap1.xml file, and open it in EditPlus, I see a rectangular dummy symbol as the first line. Just get rid of that and you should be fine.
