[fix] allow $store to be used with changing values including nullish values

Author: ramonsnir

src/runtime/internal/utils.ts

@@ -66,6 +66,9 @@ export function validate_store(store, name) {
 
 export function subscribe(store, ...callbacks) {
 	if (store == null) {
+		for (const callback of callbacks) {
+			callback(store);
+		}
 		return noop;
 	}
 	const unsub = store.subscribe(...callbacks);

test/runtime/samples/store-auto-subscribe-removed-store/_config.js

@@ -0,0 +1,19 @@
+import { writable } from '../../../../store';
+
+export default {
+	html: `
+		<p>undefined</p>
+	`,
+	async test({ assert, component, target }) {
+		component.store = writable('foo');
+		await Promise.resolve();
+		assert.htmlEqual(target.innerHTML, `
+			<p>foo</p>
+		`);
+		component.store = undefined;
+		await Promise.resolve();
+		assert.htmlEqual(target.innerHTML, `
+			<p>undefined</p>
+		`);
+	}
+};

test/runtime/samples/store-auto-subscribe-removed-store/main.svelte

@@ -0,0 +1,5 @@
+<script>
+	export let store;
+</script>
+
+<p>{$store}</p>