Investigate commit_id handling in DraftsController & API

In https://gitlab.com/gitlab-org/gitlab/-/blob/352f6d27474b04966192c44b5b1e63e844a52b59/app/controllers/projects/merge_requests/drafts_controller.rb we are tapping to cast params[commit_id] to nil when the incoming value is undefined. This was originally add 4+ years ago, and can potentially be removed from DraftsController as well as the Draft Notes API