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
naive_datetime = DEFAULT_DATE.replace(tzinfo=None)
        op_no_dag = DummyOperator(
            task_id='test_task_naive_datetime', start_date=naive_datetime, end_date=naive_datetime
        )
        self.assertTrue(op_no_dag.start_date.tzinfo)
        self.assertTrue(op_no_dag.end_date.tzinfo)
    def test_set_dag(self):
        """
        Test assigning Operators to Dags, including deferred assignment
        """
        dag = DAG('dag', start_date=DEFAULT_DATE)
        dag2 = DAG('dag2', start_date=DEFAULT_DATE)
        op = DummyOperator(task_id='op_1', owner='test')
        
        self.assertFalse(op.has_dag())
        self.assertRaises(AirflowException, getattr, op, 'dag')
        
        with self.assertRaises(TypeError):
            op.dag = 1
        op.dag = dag
        
        with self.assertRaises(AirflowException):
            op.dag = dag2
        
        op.dag = dag
        self.assertIs(op.dag, dag)
        self.assertIn(op, dag.tasks)
    def test_infer_dag(self):