list_widget: Check length of JQuery collection instead of collection

itself.

We were checking for the undefined value of `$html_item` but this
variable is always true since `$.find` never returns an undefined value
and it returns a JQuery collection instead.

So we need to check the length of that JQuery collection here to make
sure that the condition works as expected.

web/src/list_widget.ts

@@ -287,7 +287,7 @@ export function create<Key = unknown, Item = Key>(
                 return;
             }
             const $html_item = meta.$scroll_container.find(opts.html_selector(item));
-            if (!$html_item) {
+            if ($html_item.length === 0) {
                 // We don't have the item in the current scroll container; it'll be
                 // rendered with updated data when it is scrolled to.
                 return;

web/tests/list_widget.test.js

@@ -674,7 +674,10 @@ run_test("render item", () => {
         const regex = new RegExp(`\\<tr data-item=${item}\\>.*?<\\/tr\\>`);
         assert.ok(expected_queries.includes(query));
         if (query.includes(`data-item='${INITIAL_RENDER_COUNT}'`)) {
-            return undefined; // This item is not rendered, so we find nothing
+            // This item is not rendered, so we find nothing so return an empty stub.
+            return {
+                length: 0,
+            };
         }
         return {
             // Return a JQuery stub for the original HTML.
@@ -685,6 +688,7 @@ run_test("render item", () => {
                 called = true;
                 $container.$appended_data.replace(regex, new_html);
             },
+            length: 1,
         };
     };