Allow visitors to return undefined

While implementing `ts.Visitor`, it is possible to return `undefined` in order to drop a node. However, the typings do not reflect this and only allow to return `Node | Node []`.

This PR extends the typings to allow `undefined` as well.

Author: filipesilva

src/compiler/types.ts

@@ -4350,7 +4350,7 @@ namespace ts {
      */
     export type Visitor = (node: Node) => VisitResult<Node>;
 
-    export type VisitResult<T extends Node> = T | T[];
+    export type VisitResult<T extends Node> = T | T[] | undefined;
 
     export interface Printer {
         /**