add test, fix SSR

packages/svelte/src/internal/server/blocks/html.js

@@ -5,6 +5,7 @@ import { hash } from '../../../utils.js';
  * @param {string} value
  */
 export function html(value) {
-	var open = DEV ? `<!--${hash(String(value ?? ''))}-->` : '<!---->';
-	return `${open}${value}<!---->`;
+	var html = String(value ?? '');
+	var open = DEV ? `<!--${hash(html)}-->` : '<!---->';
+	return open + html + '<!---->';
 }

packages/svelte/tests/runtime-legacy/shared.ts

@@ -303,7 +303,7 @@ async function run_test_variant(
 						config.withoutNormalizeHtml === 'only-strip-comments' ? false : undefined,
 					withoutNormalizeHtml: !!config.withoutNormalizeHtml
 				});
-			} else if (config.html) {
+			} else if (config.html !== undefined) {
 				assert_html_equal_with_options(target.innerHTML, config.html, {
 					preserveComments:
 						config.withoutNormalizeHtml === 'only-strip-comments' ? false : undefined,
@@ -369,7 +369,7 @@ async function run_test_variant(
 				assert.fail('Expected a runtime error');
 			}
 
-			if (config.html) {
+			if (config.html !== undefined) {
 				flushSync();
 				assert_html_equal_with_options(target.innerHTML, config.html, {
 					preserveComments:

packages/svelte/tests/runtime-runes/samples/html-tag-empty/_config.js

@@ -0,0 +1,5 @@
+import { test } from '../../test';
+
+export default test({
+	html: ''
+});

packages/svelte/tests/runtime-runes/samples/html-tag-empty/main.svelte

@@ -0,0 +1 @@
+{@html undefined}