Auto merge of #29646 - CYBAI:fix-classname-filter, r=jdm

Fix checking for empty set in getElementsByClassName

Because empty set with `.all(predicate)` will always return `true`, it will result in always filtering wrong elements when the classes is empty set.

Thus, we return earlier with `always_empty` HTMLCollection in the empty classes case so that we can avoid filtering on empty sets.

With adding the checking, we can fix the failures in `/dom/nodes/getElementsByClassName-empty-set.html`.

---

- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)
- [x] There are tests for these changes

components/script/dom/htmlcollection.rs

@@ -252,11 +252,17 @@ impl HTMLCollection {
                 let case_sensitivity = document_from_node(elem)
                     .quirks_mode()
                     .classes_and_ids_case_sensitivity();
+
                 self.classes
                     .iter()
                     .all(|class| elem.has_class(class, case_sensitivity))
             }
         }
+
+        if classes.is_empty() {
+            return HTMLCollection::always_empty(window, root);
+        }
+
         let filter = ClassNameFilter { classes: classes };
         HTMLCollection::create(window, root, Box::new(filter))
     }