Add pass through to stripHtml for undefined and null inputs

What does this MR do?

Backport changes made in EE. See https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/5452/diffs#note_71174271

Why was this MR needed?

stripHtml is sometime called on dynamic value that can be null or undefined.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?