fix: correctly update tweened store initialized with nullish value (#10356)

Co-authored-by: Rich Harris <rich.harris@vercel.com>

Author: Rich-Harris

.changeset/quiet-timers-speak.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly update tweened store initialized with nullish value

packages/svelte/src/motion/tweened.js

@@ -89,11 +89,12 @@ export function tweened(value, defaults = {}) {
 	 * @param {import('./private').TweenedOptions<T>} [opts]
 	 */
 	function set(new_value, opts) {
+		target_value = new_value;
+
 		if (value == null) {
 			store.set((value = new_value));
 			return Promise.resolve();
 		}
-		target_value = new_value;
 
 		/** @type {import('../internal/client/types').Task | null} */
 		let previous_task = task;

packages/svelte/tests/motion/test.ts

@@ -26,5 +26,15 @@ describe('motion', () => {
 			size.set(100, { duration: 0 });
 			assert.equal(get(size), 100);
 		});
+
+		it('updates correctly when initialized with a `null`-ish value', () => {
+			const size = tweened(undefined as unknown as number, { duration: 0 });
+
+			size.set(10);
+			assert.equal(get(size), 10);
+
+			size.update((v) => v + 10);
+			assert.equal(get(size), 20);
+		});
 	});
 });