Fix MR notes not showing author badges

What does this MR do and why?

This MR fixes MR notes not showing author badges when signed out (#379375 - closed) by adding a null check ?.. We also add some missing tests.

Screenshots or screen recordings

Before (on master)	After (this MR)

How to set up and validate locally

1. Don't sign into local GDK
2. Visit a public MR with notes

Expectation:

• Note author badges are visible
• No console errors about reading properties of null should be thrown

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #379375 (closed)