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/gpui_macros/src/derive_render.rs

@@ -12,7 +12,7 @@ pub fn derive_render(input: TokenStream) -> TokenStream {
         #where_clause
         {
             fn render(&mut self, _cx: &mut gpui::ViewContext<Self>) -> impl gpui::Element {
-                ()
+                gpui::Empty
             }
         }
     };