Discussions API returns a 400 Bad Request when a quick action is only supplied

The Discussions API (https://docs.gitlab.com/ee/api/discussions.html) returns a 400 Bad Request when a quick action is only supplied as the note body:

curl -v -X POST -H "PRIVATE-TOKEN: <PAT>" "https://gitlab.example.com/api/v4/projects/1/merge_requests/20/discussions" \
  --form 'position[position_type]=text' \
  --form 'position[base_sha]=fbb2fdff97f852d80fd66d28e99c7464ddc82279' \
  --form 'position[head_sha]=b819d7d9e6b9e8ec4b2501ec935645b09b67f22a' \
  --form 'position[start_sha]=fbb2fdff97f852d80fd66d28e99c7464ddc82279' \
  --form 'position[new_path]=README.md' 
  --form 'position[old_path]=README.md' 
  --form 'position[new_line]=23' --form 'body=/spend 1d' 
  --form 'position[line_range][start][line_code]=b4974ff205e1c48c35fc404f9a7c4a20f657f244_0_23' 
  --form 'position[line_range][start][type]=new' --form 'position[line_range][end][line_code]=b4974ff205e1c48c35fc404f9a7c4a20f657f244_0_23' 
  --form 'position[line_range][end][type]=new'

{"message": "400 Bad request - Note {}"}

However, the quick action actually succeeded. We should not treat this as an error.