For xcoms pushed from different tasks, having the same execution_date and in the same Dag, the result of the xcom_pull with key set as None is not deterministic in its order

**Apache Airflow version**: 2.0


**Kubernetes version (if you are using kubernetes)** (use `kubectl version`):

**Environment**:

- **Cloud provider or hardware configuration**:
- **OS** (e.g. from /etc/os-release):
- **Kernel** (e.g. `uname -a`):
- **Install tools**:
- **Others**:

**What happened**:
When tasks in the same dag push Xcoms, the result of the xcom_pull comes in any order. The order is not deterministic.

**What you expected to happen**:

I expected the order to be deterministic.

**How to reproduce it**:

Run the provided example_xcom dag in the repository multiple times


tests/models/test_taskinstance.py
task_id='op',
            email='test@test.test',
            on_success_callback=callback_wrapper.success_handler,
            dag=dag,
        )
        ti = TI(task=task, execution_date=datetime.datetime.now())
        ti.state = State.RUNNING
        session = settings.Session()
        session.merge(ti)
        dag.create_dagrun(
            execution_date=ti.execution_date,
            state=State.RUNNING,
            run_type=DagRunType.SCHEDULED,
            session=session,
        )
        session.commit()
        callback_wrapper.wrap_task_instance(ti)
        ti._run_raw_task()
        self.assertTrue(callback_wrapper.callback_ran)
        self.assertEqual(callback_wrapper.task_state_in_callback, State.RUNNING)
        ti.refresh_from_db()
        self.assertEqual(ti.state, State.SUCCESS)
    @staticmethod
    def _test_previous_dates_setup(
        schedule_interval: Union[str, datetime.timedelta, None], catchup: bool, scenario: List[str]
    ) -> list:
        dag_id = 'test_previous_dates'
        dag = models.DAG(dag_id=dag_id, schedule_interval=schedule_interval, catchup=catchup)
        task = DummyOperator(task_id='task', dag=dag, start_date=DEFAULT_DATE)
        def get_test_ti(session, execution_date: pendulum.DateTime, state: str) -> TI: