fix: treat nullish expression as empty string (#15901)

* fix: treat nullish expression as empty string

* fix

Author: Rich-Harris

.changeset/blue-badgers-play.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: treat nullish expression as empty string

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

@@ -77,7 +77,7 @@ export function build_template_chunk(
 				// If we have a single expression, then pass that in directly to possibly avoid doing
 				// extra work in the template_effect (instead we do the work in set_text).
 				if (evaluated.is_known) {
-					value = b.literal(evaluated.value);
+					value = b.literal((evaluated.value ?? '') + '');
 				}
 
 				return { value, has_state };
@@ -96,7 +96,7 @@ export function build_template_chunk(
 			}
 
 			if (evaluated.is_known) {
-				quasi.value.cooked += evaluated.value + '';
+				quasi.value.cooked += (evaluated.value ?? '') + '';
 			} else {
 				if (!evaluated.is_defined) {
 					// add `?? ''` where necessary

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

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

packages/svelte/tests/runtime-runes/samples/nullish-empty-string/main.svelte

@@ -0,0 +1 @@
+[{undefined ?? null}]

packages/svelte/tests/snapshot/samples/purity/_expected/client/index.svelte.js

@@ -8,7 +8,7 @@ export default function Purity($$anchor) {
 	var fragment = root();
 	var p = $.first_child(fragment);
 
-	p.textContent = 0;
+	p.textContent = '0';
 
 	var p_1 = $.sibling(p, 2);