Part 3: Make string attributes non-nullable

Differential Revision: https://phabricator.services.mozilla.com/D273237

bugzilla-url: https://bugzilla.mozilla.org/show_bug.cgi?id=2000862
gecko-commit: 72ca96b3163817d954c505370c5166d6283bf5de
gecko-reviewers: smaug, webidl

Author: saschanaz

notifications/constructor-basic.https.html

@@ -43,4 +43,12 @@
         );
     }
 }, "constructing a notification with a NotificationOptions dictionary correctly sets and reflects the silent attribute.");
+
+test(() => {
+  const notification = new Notification("a");
+  assert_equals(notification.lang, "", "lang should default to an empty string");
+  assert_equals(notification.body, "", "body should default to an empty string");
+  assert_equals(notification.tag, "", "tag should default to an empty string");
+  assert_equals(notification.icon, "", "icon should default to an empty string");
+}, "String options should default to empty strings");
 </script>