fix: ensure proxy owner set always has 1 or more members (#10577)

* fix missing owners

* regression test

---------

Co-authored-by: Rich Harris <[email protected]>

Author: Rich-Harris

.changeset/sour-bags-fail.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: inherit ownerlessness when creating child proxies

packages/svelte/src/internal/client/proxy.js

@@ -84,7 +84,7 @@ export function proxy(value, immutable = true, owners) {
 							? // @ts-expect-error
 								new Set([current_component_context.function])
 							: null
-						: new Set(owners);
+						: owners && new Set(owners);
 			}
 
 			return proxy;

packages/svelte/tests/runtime-runes/samples/non-local-mutation-inherited-owner-2/_config.js

@@ -0,0 +1,37 @@
+import { test } from '../../test';
+
+/** @type {typeof console.warn} */
+let warn;
+
+/** @type {any[]} */
+let warnings = [];
+
+export default test({
+	html: `<button>clicks: 0</button>`,
+
+	compileOptions: {
+		dev: true
+	},
+
+	before_test: () => {
+		warn = console.warn;
+
+		console.warn = (...args) => {
+			warnings.push(...args);
+		};
+	},
+
+	after_test: () => {
+		console.warn = warn;
+		warnings = [];
+	},
+
+	async test({ assert, target }) {
+		const btn = target.querySelector('button');
+		await btn?.click();
+
+		assert.htmlEqual(target.innerHTML, `<button>clicks: 1</button>`);
+
+		assert.deepEqual(warnings, []);
+	}
+});

packages/svelte/tests/runtime-runes/samples/non-local-mutation-inherited-owner-2/main.svelte

@@ -0,0 +1,9 @@
+<script>
+	import { global } from './state.svelte.js';
+
+	global.a = { b: 0 };
+</script>
+
+<button onclick={() => global.a.b += 1}>
+	clicks: {global.a.b}
+</button>

packages/svelte/tests/runtime-runes/samples/non-local-mutation-inherited-owner-2/state.svelte.js

@@ -0,0 +1 @@
+export let global = $state({});