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 = DummyOperator(task_id='op')
        ti = TI(task=task, execution_date=datetime.datetime.now())
        params = {"override": False}
        dag_run = DagRun()
        ti.overwrite_params_with_dag_run_conf(params, dag_run)
        self.assertEqual(False, params["override"])
    @patch('airflow.models.taskinstance.send_email')
    def test_email_alert(self, mock_send_email):
        dag = models.DAG(dag_id='test_failure_email')
        task = BashOperator(
            task_id='test_email_alert', dag=dag, bash_command='exit 1', start_date=DEFAULT_DATE, email='to'
        )
        ti = TI(task=task, execution_date=timezone.utcnow())
        try:
            ti.run()
        except AirflowException:
            pass
        (email, title, body), _ = mock_send_email.call_args
        self.assertEqual(email, 'to')
        self.assertIn('test_email_alert', title)
        self.assertIn('test_email_alert', body)
        self.assertIn('Try 1', body)
    @conf_vars(
        {
            ('email', 'subject_template'): '/subject/path',
            ('email', 'html_content_template'): '/html_content/path',
        }
    )
    @patch('airflow.models.taskinstance.send_email')
    def test_email_alert_with_config(self, mock_send_email):