Which is it? " " or ".". That's an easy way to confuse people whilst trying to be complete. This is supposed to be a 101 class.
. is the designation for root in DNS but it is not spelt out except where it is!
For example: In a (BIND) zone file you terminate an entry with . otherwise it is considered relative to the "current" domain. When you type into your browser something like www.example.com you don't include a trailing dot.
Finally, the empty string was shown like this: " ". That's a space.
"The name of the root is the empty string (" ") generally denoted with a dot (.):"
... and left.