Replaced usage of toBeFalsy with toBeUndefined

What does this MR do and why?

Replaced usage of toBeFalsy with toBeUndefined in spec/frontend/vue_shared/components/markdown/header_spec.js

Resolves the issue

Screenshots or screen recordings

How to set up and validate locally

1. Navigate to gitlab directory

2. Run

   yarn jest spec/frontend/vue_shared/components/markdown/header_spec.js

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 #369814 (closed)