fix(material/tree): refactor again

refactored isExpanded function again

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,
 })
@@ -388,7 +388,10 @@ export class CdkTreeNode<T, K = T> implements FocusableOption, OnDestroy, OnInit
     return false;
   }
 
-  get isExpanded(): boolean {
+  get isExpanded(): boolean | null {
+    if (this.isLeafNode) {
+      return null;
+    }
     return this._tree.treeControl.isExpanded(this._data);
   }