You can call extension methods on null elements. It’s obvious when you think about it: its a normal static method where you specify its first parameter with this.
Notice that if it was normal instance method (i.e. defined in the System.String class) I would have to check if my string instance is not null, before calling it, or I would receive runtime NullReferenceException. Now I can move that check to the method, where it belongs, and make my code cleaner. Pretty slick 😉