Prevent new note notifications from Ghost user

When the note author is already deleted, we should skip creation of
notifications and events

app/workers/new_note_worker.rb

@@ -13,7 +13,7 @@ class NewNoteWorker # rubocop:disable Scalability/IdempotentWorker
   # rubocop: disable CodeReuse/ActiveRecord
   def perform(note_id, _params = {})
     if note = Note.find_by(id: note_id)
-      NotificationService.new.new_note(note) unless note.skip_notification?
+      NotificationService.new.new_note(note) unless note.skip_notification? || note.author.ghost?
       Notes::PostProcessService.new(note).execute
     else
       Gitlab::AppLogger.error("NewNoteWorker: couldn't find note with ID=#{note_id}, skipping job")

changelogs/unreleased/prevent-notifications-from-ghost-user.yml

+---
+title: Skip new note notifications when author is deleted
+merge_request: 53699
+author:
+type: changed

spec/workers/new_note_worker_spec.rb

@@ -65,4 +65,14 @@ RSpec.describe NewNoteWorker do
       subject.perform(note.id)
     end
   end
+
+  context 'when Note author has been deleted' do
+    let_it_be(:note) { create(:note, author: User.ghost) }
+
+    it "does not call NotificationService" do
+      expect(NotificationService).not_to receive(:new)
+
+      described_class.new.perform(note.id)
+    end
+  end
 end