Handle emptyText in user_avatar_list

What does this MR do?

This updates the shared component user_avatar_list to handle it's empty text. This is needed for the EE MR https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9176.

What are the relevant issue numbers?

• gitlab-ee#1979

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated via this MR
• Tests added for this feature/bug
• Tested in all supported browsers
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer