Fixes draft notes API not being able to create diff file draft notes

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/157944



Merged-by: Michał Wielich <mwielich@gitlab.com>
Approved-by: Naman Jagdish Gala <ngala@gitlab.com>
Approved-by: Michał Wielich <mwielich@gitlab.com>
Co-authored-by: Phil Hughes <me@iamphill.com>