[Web] tolerate empty e.target in addTargetBlankToExternalLinks (#2555)

## Motivation for the change, related issues

A user reported the following error coming from the
`addTargetBlankToExternalLinks` function:

```
JavaScript Error: TypeError: e.target.closest is not a function https://playground.wordpress.net/scope:0.7531680412299814/wp-admin/post.php?post=5&action=edit:570:24
```

It could happen when another event listener removes the `e.target` node
before the event bubbles up to the `document` node.

This PR makes sure an empty `e.target` is tolerated and short-circuits
without an error.

cc @draganescu

Author: adamziel

packages/playground/remote/src/lib/playground-mu-plugin/0-playground.php

@@ -120,15 +120,15 @@ function addTargetBlank(a) {
 			// Set target="_blank" for external links when clicked.
 			// This covers links that are added after the page has loaded.
 			document.addEventListener('click', e => {
-				const a = e.target.closest('a[href]');
+				const a = e.target?.closest('a[href]');
 				if (!a) return;
 				addTargetBlank(a);
 			});
 
 			// Also handle focus events to cover keyboard navigation on
 			// links that are added after the page has loaded.
 			document.addEventListener('focus', e => {
-				const a = e.target.closest('a[href]');
+				const a = e.target?.closest('a[href]');
 				if (!a) return;
 				addTargetBlank(a);
 			}, true);