Fixed avatar not displaying in issue boards

This happens when Gravatar is disabled in the admin settings, the avatar is returned as null & then frontend didn't do anything about it.

Closes #31428

app/assets/javascripts/boards/models/user.js

 /* eslint-disable no-unused-vars */
+import defaultAvatar from '../utils/default_avatar';
 class ListUser {
   constructor(user) {
     this.id = user.id;
     this.name = user.name;
     this.username = user.username;
-    this.avatar = user.avatar_url;
+    this.avatar = user.avatar_url || defaultAvatar();
   }
 }

app/assets/javascripts/boards/utils/default_avatar.js

+export default () => document.getElementById('board-app').dataset.defaultAvatar;

app/helpers/boards_helper.rb

@@ -9,6 +9,7 @@ module BoardsHelper
       issue_link_base: namespace_project_issues_path(@project.namespace, @project),
       root_path: root_path,
       bulk_update_path: bulk_update_namespace_project_issues_path(@project.namespace, @project),
+      default_avatar: image_path(default_avatar)
     }
   end
 end

changelogs/unreleased/issue-boards-no-avatar.yml

+---
+title: Fixed avatar not display on issue boards when Gravatar is disabled
+merge_request:
+author: