fix(material/tree): refactor isLeafNode function

refactored isLeafNode function to make more sense

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',
+    '[attr.aria-expanded]': 'isLeafNode ? null : isExpanded',
   },
   standalone: true,
 })
@@ -368,23 +368,27 @@ export class CdkTreeNode<T, K = T> implements FocusableOption, OnDestroy, OnInit
   }
   protected _data: T;
 
-  /* If leaf node, do not assign aria-expanded attribute */
-  get isLeafNode(): boolean | null {
+  /* If leaf node, return true to not assign aria-expanded attribute */
+  get isLeafNode(): boolean {
     // If flat tree node data returns false for expandable property, it's a leaf node
     if (
       this._tree.treeControl.isExpandable !== undefined &&
       !this._tree.treeControl.isExpandable(this._data)
     ) {
-      return null;
+      return true;
 
       // If nested tree node data returns 0 descendants, it's a leaf node
     } else if (
       this._tree.treeControl.isExpandable === undefined &&
       this._tree.treeControl.getDescendants(this._data).length === 0
     ) {
-      return null;
+      return true;
     }
 
+    return false;
+  }
+
+  get isExpanded(): boolean {
     return this._tree.treeControl.isExpanded(this._data);
   }