Fix work item notes failing to load

What does this MR do and why?

gitlab-com/gl-infra/production#17436 (closed)

Fix work item notes failing to load

When the user who assigned a label is deleted, the corresponding resource_label_events DB record sets user_id to null.

The author_id field of the note GraphQL type should be nullable to conform to the domain model.

Changelog: fixed

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.