Creating MR comment without a note returns status code 400 (Bad request)

Creating a comment to an existing merge request via API without providing a note
returns a status code 400 now, suggesting a bad request. The reason for this
is the resource itself (MR) exists but the required property is not set.

lib/api/merge_requests.rb

@@ -128,6 +128,9 @@ module Gitlab
         if note.save
           present note, with: Entities::MRNote
         else
+          if note.errors[:note].any?
+            error!(note.errors[:note], 400)
+          end
           not_found!
         end
       end