fix(curriculum): add assert to prevent empty editor pass in todo list (#64193)

curriculum/challenges/english/blocks/lab-stylized-to-do-list/66c051d13a6a20255a963695.md

@@ -100,6 +100,7 @@ The `id` and `for` attributes of the `input` and `label` elements pairs, should
 
 ```js
 const labels = document.querySelectorAll("label");
+assert.isNotEmpty(labels);
 
 document.querySelectorAll("input").forEach((input, index) => {
   assert.equal(input.id, labels[index].htmlFor);