don't return undefined from _executePrepareCallHierarchy

src/vs/workbench/contrib/callHierarchy/browser/callHierarchy.ts

@@ -162,7 +162,7 @@ CommandsRegistry.registerCommand('_executePrepareCallHierarchy', async (accessor
 	try {
 		const model = await CallHierarchyModel.create(textModel, position, CancellationToken.None);
 		if (!model) {
-			return undefined;
+			return [];
 		}
 		//
 		_models.set(model.id, model);