fix: don't throw for undefined non delegated event handlers

Author: paoloricciuti

.changeset/clever-cherries-hear.md

@@ -0,0 +1,5 @@
+---
+'svelte': patch
+---
+
+fix: don't throw for `undefined` non delegated event handlers

packages/svelte/src/internal/client/dom/elements/events.js

@@ -63,7 +63,7 @@ export function create_event(event_name, dom, handler, options) {
 		}
 		if (!event.cancelBubble) {
 			return without_reactive_context(() => {
-				return handler.call(this, event);
+				return handler?.call(this, event);
 			});
 		}
 	}

packages/svelte/tests/runtime-runes/samples/undefined-event-handler/_config.js

@@ -0,0 +1,9 @@
+import { ok, test } from '../../test';
+
+export default test({
+	async test({ target }) {
+		const button = target.querySelector('button');
+		ok(button);
+		button.dispatchEvent(new window.MouseEvent('mouseenter'));
+	}
+});

packages/svelte/tests/runtime-runes/samples/undefined-event-handler/main.svelte

@@ -0,0 +1 @@
+<button onmouseenter={undefined}></button>