FIX: ensures global notices are destroyed on post created (#28684)

Prior to this fix we could exit early if tags was `[]` as `tags && (tags & post.topic.tags.map(&:name)).empty?` would have returned true. This commit ensures it's not the case anymore and adds a test for it.

Co-Authored-By: Martin Brennan <mjrbrennan@gmail.com>

plugins/automation/spec/fabricators/user_global_notice_fabricator.rb

@@ -0,0 +1,8 @@
+# frozen_string_literal: true
+
+Fabricator(:user_global_notice, from: DiscourseAutomation::UserGlobalNotice) do
+  user_id { Fabricate(:user).id }
+  notice "This is an important notice"
+  level "info"
+  identifier "foo"
+end