feat(tests): Updated toBeTruthy to toBeDefined in diff_gutter_avatars_spec.js

What does this MR do and why?

• This MR replaces toBeFalsy with toBe(false)
• This MR fixes #369756 (closed)

Screenshots or screen recordings

How to set up and validate locally

1. yarn
2. yarn jest spec/frontend/diffs/components/diff_gutter_avatars_spec.js

MR acceptance checklist

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