fix: handle sole empty expression tags

When there's only a single expression tag and its value evaluates to the empty string, special handling is needed to create and insert a text node
fixes #10426

Author: dummdidumm

.changeset/silent-apes-report.md

@@ -0,0 +1,5 @@
+---
+"svelte": patch
+---
+
+fix: handle sole empty expression tags

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

@@ -201,7 +201,7 @@ export function child_frag(node, is_text) {
 			return text;
 		}
 
-		if (first_node !== null) {
+		if (first_node) {
 			return capture_fragment_from_node(first_node);
 		}
 

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

@@ -212,11 +212,22 @@ const space_template = template(' ', false);
 const comment_template = template('<!>', true);
 
 /**
- * @param {null | Text | Comment | Element} anchor
+ * @param {Text | Comment | Element | null} anchor
  */
 /*#__NO_SIDE_EFFECTS__*/
 export function space(anchor) {
-	return open(anchor, true, space_template);
+	/** @type {any} */
+	var node = open(anchor, true, space_template);
+	// if an {expression} is empty during SSR, there might be no
+	// text node to hydrate (or a anchor comment is falsely detected instead)
+	//  — we must therefore create one
+	if (current_hydration_fragment !== null && node?.nodeType !== 3) {
+		node = document.createTextNode('');
+		// @ts-ignore in this case the anchor should always be a comment,
+		// if not something more fundamental is wrong and throwing here is better to bail out early
+		anchor.parentElement.insertBefore(node, anchor.nextSibling);
+	}
+	return node;
 }
 
 /**

packages/svelte/tests/hydration/samples/text-empty/_after.html

@@ -0,0 +1 @@
+<!--ssr:0-->x<!--ssr:0-->

packages/svelte/tests/hydration/samples/text-empty/_before.html

@@ -0,0 +1 @@
+<!--ssr:0--><!--ssr:0-->

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

@@ -0,0 +1,3 @@
+import { test } from '../../test';
+
+export default test({});

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

@@ -0,0 +1,5 @@
+<script>
+    let x = typeof window === 'undefined' ? '' : 'x'
+</script>
+
+{x}