fix: correctly hydrate empty raw blocks (#12979)

* fix: correctly hydrate empty raw blocks

* lint

* Update packages/svelte/src/internal/client/dom/blocks/html.js

---------

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

.changeset/shiny-wombats-argue.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: correctly hydrate empty raw blocks

packages/svelte/src/internal/client/dom/blocks/html.js

@@ -46,15 +46,20 @@ export function html(node, get_value, svg, mathml, skip_warning) {
 
 	var value = '';
 
-	/** @type {Effect | null} */
+	/** @type {Effect | undefined} */
 	var effect;
 
 	block(() => {
-		if (value === (value = get_value())) return;
+		if (value === (value = get_value())) {
+			if (hydrating) {
+				hydrate_next();
+			}
+			return;
+		}
 
-		if (effect) {
+		if (effect !== undefined) {
 			destroy_effect(effect);
-			effect = null;
+			effect = undefined;
 		}
 
 		if (value === '') return;

packages/svelte/tests/hydration/samples/raw-empty/_config.js

@@ -0,0 +1,15 @@
+import { test } from '../../test';
+
+export default test({
+	server_props: {
+		html: '<div></div>'
+	},
+
+	props: {
+		html: '<div></div>'
+	},
+
+	test(assert, target) {
+		assert.htmlEqual(target.innerHTML, '<div></div>');
+	}
+});

packages/svelte/tests/hydration/samples/raw-empty/main.svelte

@@ -0,0 +1 @@
+<div>{@html ''}</div>