Change window_min_size from Size<Pixels> to Option<Size<Pixels>> (#13501)

Now we can set `window_min_size` to `None` instead of `Size::default()`.
I think this makes more sense.

Release Notes:

- N/A

crates/gpui/examples/window_positioning.rs

@@ -51,7 +51,7 @@ fn main() {
                     kind: WindowKind::PopUp,
                     is_movable: false,
                     app_id: None,
-                    window_min_size: Size::default(),
+                    window_min_size: None,
                 }
             };