fix(material/tree): aria-expanded attribute should not appear in the leaf node

Fixed an issue to where leaf nodes were wrongly being assigned the aria-expanded attribute, which could confuse screen reader users.

Fixes #21922

Author: DBowen33

src/cdk/tree/tree.ts

@@ -321,7 +321,7 @@ export class CdkTree<T, K = T> implements AfterContentChecked, CollectionViewer,
   exportAs: 'cdkTreeNode',
   host: {
     'class': 'cdk-tree-node',
-    '[attr.aria-expanded]': 'isLeafNode ? null : isExpanded',
+    '[attr.aria-expanded]': 'isExpanded',
   },
   standalone: true,
 })