Return null attachment when there is none

app/assets/javascripts/notes/components/issue_note_body.vue

@@ -112,7 +112,7 @@
       :toggle-award-path="note.toggle_award_path"
       />
     <issue-note-attachment
-      v-if="note.attachment.url"
+      v-if="note.attachment"
       :attachment="note.attachment"
       />
   </div>

app/serializers/note_entity.rb

@@ -53,7 +53,7 @@ class NoteEntity < API::Entities::Note
     end
   end
 
-  expose :attachment, using: NoteAttachmentEntity
+  expose :attachment, using: NoteAttachmentEntity, if: -> (note, _) { note.attachment? }
   expose :delete_attachment_path, if: -> (note, _) { note.attachment? } do |note|
     delete_attachment_project_note_path(note.project, note)
   end