Added undefined example (#8899)

Into section `Booleans, Null, and Undefined Are Ignored` I've added example with undefined.

docs/docs/jsx-in-depth.md

@@ -381,6 +381,8 @@ Children passed to a custom component can be anything, as long as that component
 
 <div>{null}</div>
 
+<div>{undefined}</div>
+
 <div>{true}</div>
 ```