Hide the chat message editor when there is no active chat (#7441)

This PR makes it so the chat message editor is hidden when not in an
active chat.

Release Notes:

- Changed the chat message editor to be hidden when not in an active
chat.

crates/collab_ui/src/chat_panel.rs

@@ -589,7 +589,8 @@ impl Render for ChatPanel {
                     )
                 }
             }))
-            .child(
+            .children(
+                Some(
                     h_flex()
                         .when(!self.is_scrolled_to_bottom, |el| {
                             el.border_t_1().border_color(cx.theme().colors().border)
@@ -609,6 +610,8 @@ impl Render for ChatPanel {
                             }
                         }),
                 )
+                .filter(|_| self.active_chat.is_some()),
+            )
             .into_any()
     }
 }