Seems ALT+0173 works here as a "blank" character. I'm not sure of its exact purpose, but I've never seen it dealt with and often use it as "nothing". The only solution I've seen to properly sanitising Unicode characters is just to disable them entirely and print their name.
If you break my “typographer’s quotes” by overzealously sanitizing when you don’t absolutely need to, you’ll end up 6' under a † if you know what I mean. ;-)
While it makes sense to make usernames consist of only alphanumeric characters (something people are used to - thanks to email), I don't see why you'd need to "sanitize" Unicode characters.