You're right. Length == 0 is faster than IsNullOrEmpty. However, you'll need to add a c == null check here too since the "Convert" method is sometimes called with text directly from GitHub and that text is sometimes null. Instead of putting that check everywhere outside of this method it's easier to just check it here and return the empty string too.

tl;dr; Looks good. Just add a null check and I'll approve it. Thanks!