Show auto-generated avatars in Groups dashboard tree for Groups without avatars

What does this MR do?

Shows auto-generated avatars to groups without avatars (similar to Projects)

Why was this MR needed?

It makes UX for Groups consistent to that of Projects.

Screenshots

Groups List

Before	After

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• 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
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#35408 (moved)