gpui: Don't impl IntoElement on () (#8555)

Although it's kinda cute, rust makes it too easy to accidentally return
() from a function.

/cc @nathansobo

Release Notes:

- N/A

crates/search/src/buffer_search.rs

@@ -1477,7 +1477,7 @@ mod tests {
                 buffer_text,
             )
         });
-        let window = cx.add_window(|_| ());
+        let window = cx.add_window(|_| gpui::Empty);
 
         let editor = window.build_view(cx, |cx| Editor::for_buffer(buffer.clone(), None, cx));