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
raise
            ti.refresh_from_db()
            self.assertEqual(ti.state, expected_state)
            self.assertEqual(ti._try_number, expected_try_number)
            self.assertEqual(ti.try_number, expected_try_number + 1)
            self.assertEqual(ti.start_date, expected_start_date)
            self.assertEqual(ti.end_date, expected_end_date)
            self.assertEqual(ti.duration, expected_duration)
            trs = TaskReschedule.find_for_task_instance(ti)  
            self.assertEqual(len(trs), expected_task_reschedule_count)
        date1 = timezone.utcnow()
        done, fail = False, False
        run_ti_and_assert(date1, date1, date1, 0, State.UP_FOR_RESCHEDULE, 0, 1)
        
        dag.clear()
        ti.refresh_from_db()
        self.assertEqual(ti.state, State.NONE)
        self.assertEqual(ti._try_number, 0)
        
        trs = TaskReschedule.find_for_task_instance(ti)  
        self.assertFalse(trs)
    def test_depends_on_past(self):
        dag = DAG(dag_id='test_depends_on_past', start_date=DEFAULT_DATE)
        task = DummyOperator(
            task_id='test_dop_task',
            dag=dag,
            depends_on_past=True,
        )
        dag.clear()
        run_date = task.start_date + datetime.timedelta(days=5)