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

crates/assistant2/src/ui/chat_message.rs

@@ -69,26 +69,25 @@ impl RenderOnce for ChatMessage {
         // Clamp the message height to exactly 1.5 lines when collapsed.
         let collapsed_height = content_padding.to_pixels(cx.rem_size()) + cx.line_height() * 1.5;
 
-        let tools_used = self
-            .tools_used
-            .map(|attachment| div().mt_3().child(attachment));
-
-        let content = self.message.map(|message| {
-            div()
-                .overflow_hidden()
-                .w_full()
-                .p(content_padding)
-                .rounded_lg()
-                .when(self.collapsed, |this| this.h(collapsed_height))
-                .bg(cx.theme().colors().surface_background)
-                .child(message)
-                .children(tools_used)
-        });
-
         v_flex()
             .gap_1()
             .child(ChatMessageHeader::new(self.player))
-            .child(h_flex().gap_3().child(collapse_handle).children(content))
+            .when(self.message.is_some() || self.tools_used.is_some(), |el| {
+                el.child(
+                    h_flex().gap_3().child(collapse_handle).child(
+                        div()
+                            .overflow_hidden()
+                            .w_full()
+                            .p(content_padding)
+                            .gap_3()
+                            .rounded_lg()
+                            .when(self.collapsed, |this| this.h(collapsed_height))
+                            .bg(cx.theme().colors().surface_background)
+                            .children(self.message)
+                            .children(self.tools_used),
+                    ),
+                )
+            })
     }
 }