assistant2: Add empty state for new threads (#21542)

This PR adds an empty state for new threads in Assistant2:

<img width="1138" alt="Screenshot 2024-12-04 at 12 17 46 PM"
src="https://github.com/user-attachments/assets/ff7b4533-d3b8-4722-bd4b-43fac6d35a77">

This is mostly just a sketch in its current state.

Release Notes:

- N/A