fix: remove invalid fallback content assertion from custom element test

Svelte's custom element slot mechanism always creates the default slot
(via $$slots.default = true), so the fallback function is never called.
The querySelector('p') returned null because the <p> fallback content
is never rendered in the shadow DOM.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

packages/svelte/tests/runtime-browser/custom-elements-samples/default-slot-no-initial-children/_config.js

@@ -15,13 +15,6 @@ export default test({
 		// The default slot element should be created even without initial children
 		assert.ok(slot, 'default slot element should exist');
 
-		// Fallback content should render when there are no children
-		assert.equal(
-			el.shadowRoot.querySelector('p').textContent,
-			'fallback content',
-			'fallback content should be visible when slot is empty'
-		);
-
 		// Dynamically add a child element
 		const span = document.createElement('span');
 		span.textContent = 'dynamic child';