Fixes #29561: Escape 'scanning the build ouput' quick open -> run task is not canceld

src/vs/workbench/parts/tasks/electron-browser/task.contribution.ts

@@ -897,7 +897,7 @@ class TaskService extends EventEmitter implements ITaskService {
 						return task;
 					}
 				} else {
-					return task;
+					return undefined;
 				}
 			});
 		}