Include null and undefined elements in flat() examples

- I wasn't sure if `null`, `undefined`, and "empty" were equivalent in the eyes of the `flat()` method, so I tried it out in the console and learned that they are not.
- I've modified the example for sparse arrays to include these values so that others don't have to run their own experiments to learn this information. 
- I also included a link to the sparse array documentation. If the way `flat()` handles `null` and `undefined` confuses the reader, I think they are likely to click on this link to educate themselves.

files/en-us/web/javascript/reference/global_objects/array/flat/index.md

@@ -61,7 +61,7 @@ arr4.flat(Infinity);
 
 ### Using flat() on sparse arrays
 
-The `flat()` method removes empty slots in arrays:
+The `flat()` method removes [empty slots](/en-US/docs/Web/JavaScript/Guide/Indexed_collections#sparse_arrays) in arrays:
 
 ```js
 const arr5 = [1, 2, , 4, 5];
@@ -70,9 +70,9 @@ console.log(arr5.flat()); // [1, 2, 4, 5]
 const array = [1, , 3, ["a", , "c"]];
 console.log(array.flat()); // [ 1, 3, "a", "c" ]
 
-const array2 = [1, , 3, ["a", , ["d", , "e"]]];
-console.log(array2.flat()); // [ 1, 3, "a", ["d", empty, "e"] ]
-console.log(array2.flat(2)); // [ 1, 3, "a", "d", "e"]
+const array2 = [1, , 3, undefined, ["a", , ["d", , "e"]], null];
+console.log(array2.flat()); // [ 1, 3, undefined, "a", ["d", empty, "e"], null ]
+console.log(array2.flat(2)); // [ 1, 3, undefined, "a", "d", "e", null ]
 ```
 
 ### Calling flat() on non-array objects