Restore rendering of assistant tool calls (#11385)

Chat message rendering was restructured in
https://github.com/zed-industries/zed/pull/11327, but it caused tool
calls not to be rendered if the assistant hadn't generated any message
text.

before:

![Screenshot 2024-05-03 at 10 02
57 PM](https://github.com/zed-industries/zed/assets/326587/2b7fd763-0c75-4690-9824-3bd37a3efef2)

after:

<img width="518" alt="Screenshot 2024-05-03 at 11 17 45 PM"
src="https://github.com/zed-industries/zed/assets/326587/34de19ba-daf2-4ac1-9fe0-f51d0ce94872">

Release Notes:

- N/A