Fix serialization of LegacyDiffNote

changelogs/unreleased/fix-diff-note.yml

+---
+title: Fix serialization of LegacyDiffNote
+merge_request:
+author:
+type: fixed

lib/api/entities.rb

@@ -701,7 +701,7 @@ module API
       expose :system?, as: :system
       expose :noteable_id, :noteable_type
 
-      expose :position, if: ->(note, options) { note.diff_note? } do |note|
+      expose :position, if: ->(note, options) { note.is_a?(DiffNote) } do |note|
         note.position.to_h
       end
 

spec/factories/notes.rb

@@ -39,6 +39,7 @@ FactoryBot.define do
 
     factory :legacy_diff_note_on_merge_request, traits: [:on_merge_request, :legacy_diff_note], class: LegacyDiffNote do
       association :project, :repository
+      position ''
     end
 
     factory :diff_note_on_merge_request, traits: [:on_merge_request], class: DiffNote do