Update onDidChangeTreeData event type, fixes #96932 (#96933)

Author: Rachel Macfarlane

src/vs/vscode.d.ts

@@ -7897,7 +7897,7 @@ declare module 'vscode' {
 		 * This will trigger the view to update the changed element/root and its children recursively (if shown).
 		 * To signal that root has changed, do not pass any argument or pass `undefined` or `null`.
 		 */
-		onDidChangeTreeData?: Event<T | undefined | null>;
+		onDidChangeTreeData?: Event<T | undefined | null | void>;
 
 		/**
 		 * Get [TreeItem](#TreeItem) representation of the `element`

src/vs/workbench/api/common/extHostTreeViews.ts

@@ -153,7 +153,7 @@ export class ExtHostTreeViews implements ExtHostTreeViewsShape {
 	}
 }
 
-type Root = null | undefined;
+type Root = null | undefined | void;
 type TreeData<T> = { message: boolean, element: T | Root | false };
 
 interface TreeNode extends IDisposable {