chore: tidy up scrolling code logic (#11630)

---------

Co-authored-by: Simon H <[email protected]>

Author: adiguba

packages/svelte/src/internal/client/dom/elements/bindings/window.js

@@ -33,9 +33,7 @@ export function bind_window_scroll(type, get_value, update) {
 
 	render_effect(() => {
 		latest_value = get_value();
-		if (latest_value === undefined) return;
-
-		if (!scrolling) {
+		if (!scrolling && latest_value != null) {
 			scrolling = true;
 			clearTimeout(timeout);
 			if (is_scrolling_x) {