Fix autocomplete initial undefined state (loading)

What does this MR do?

Show the loading state while waiting for the ajax request instead of undefined.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Before

After

Does this MR meet the acceptance criteria?

• Changelog entry added
• Tests
  • All builds are passing
• 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?

Closes #26775 (closed)