fix: coerce nullish <title> to empty string (#16863)

Author: Ocean-OS

.changeset/silly-walls-fail.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: coerce nullish `<title>` to empty string

packages/svelte/src/compiler/phases/3-transform/client/visitors/TitleElement.js

@@ -12,8 +12,19 @@ export function TitleElement(node, context) {
 		/** @type {any} */ (node.fragment.nodes),
 		context
 	);
+	const evaluated = context.state.scope.evaluate(value);
 
-	const statement = b.stmt(b.assignment('=', b.id('$.document.title'), value));
+	const statement = b.stmt(
+		b.assignment(
+			'=',
+			b.id('$.document.title'),
+			evaluated.is_known
+				? b.literal(evaluated.value)
+				: evaluated.is_defined
+					? value
+					: b.logical('??', value, b.literal(''))
+		)
+	);
 
 	if (has_state) {
 		context.state.update.push(statement);

packages/svelte/tests/runtime-runes/samples/title-nullish/_config.js

@@ -0,0 +1,7 @@
+import { test } from '../../test';
+
+export default test({
+	test({ assert, target }) {
+		assert.equal(target.ownerDocument.title, '');
+	}
+});

packages/svelte/tests/runtime-runes/samples/title-nullish/main.svelte

@@ -0,0 +1,6 @@
+<script>
+    const thing = {};
+</script>
+<svelte:head>
+    <title>{thing.thing}</title>
+</svelte:head>