Fix batch query issue when primary key is -1

What does this MR do?

A strange bug was reported where audit event logs that included events authored by non-existent users would display the wrong author name.

Reproduction instructions are included in the issue itself.

It looks like the problem was caused by batch loading audit events where the author ID was -1. By adding a guard clause to the lazy loading method to return a null object if author_id is -1, this issue is avoided.

This MR also adds an end-to-end test case that reproduces the issue.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team

Related to #288822