Add jsdoc nullable union test case to fourslash

Author: sandersn

tests/cases/fourslash/jsdocNullableUnion.ts

@@ -0,0 +1,23 @@
+///<reference path="fourslash.ts" />
+// @allowNonTsExtensions: true
+// @Filename: Foo.js
+////
+//// * @param {never | {x: string}} p1
+//// * @param {undefined | {y: number}} p2
+//// * @param {null | {z: boolean}} p3
+//// * @returns {void} nothing
+//// */
+////function f(p1, p2, p3) {
+////    p1./*1*/
+////    p2./*2*/
+////    p3./*3*/
+////}
+
+goTo.marker('1');
+verify.memberListContains("x");
+
+goTo.marker('2');
+verify.memberListContains("y");
+
+goTo.marker('3');
+verify.memberListContains("z");