fix: treat undefined and null the same for the initial input value

Fixes #14558

Author: dummdidumm

.changeset/cool-clocks-film.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: treat `undefined` and `null` the same for the initial input value

packages/svelte/src/internal/client/dom/elements/attributes.js

@@ -61,7 +61,10 @@ export function set_value(element, value) {
 	// @ts-expect-error
 	var attributes = (element.__attributes ??= {});
 	if (
-		attributes.value === (attributes.value = value) ||
+		attributes.value ===
+			(attributes.value =
+				// treat null and undefined the same for the initial value
+				value ?? undefined) ||
 		// @ts-expect-error
 		// `progress` elements always need their value set when its `0`
 		(element.value === value && (value !== 0 || element.nodeName !== 'PROGRESS'))