Add assertion to dragend null test case

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/js/sortable.test.js

@@ -106,9 +106,10 @@ describe('initSortable', () => {
 
         initSortable()
 
+        const item = list.querySelector('[data-id]')
         const endEvent = createDragEvent('dragend')
         list.dispatchEvent(endEvent)
-        // no error thrown
+        expect(item.classList.contains('opacity-50')).toBe(false)
     })
 
     it('moves element before target on dragover above midpoint', () => {