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
with create_session() as session:
            dag = DAG('test_queries', start_date=DEFAULT_DATE)
            task = DummyOperator(task_id='op', dag=dag)
            ti = TI(task=task, execution_date=datetime.datetime.now())
            ti.state = State.RUNNING
            session.merge(ti)
            dag.create_dagrun(
                execution_date=ti.execution_date,
                state=State.RUNNING,
                run_type=DagRunType.SCHEDULED,
                session=session,
            )
        with assert_queries_count(10):
            ti._run_raw_task()
    def test_operator_field_with_serialization(self):
        dag = DAG('test_queries', start_date=DEFAULT_DATE)
        task = DummyOperator(task_id='op', dag=dag)
        self.assertEqual(task.task_type, 'DummyOperator')
        
        ti = TI(task=task, execution_date=datetime.datetime.now())
        self.assertEqual(ti.operator, "DummyOperator")
        serialized_op = SerializedBaseOperator.serialize_operator(task)
        deserialized_op = SerializedBaseOperator.deserialize_operator(serialized_op)
        self.assertEqual(deserialized_op.task_type, 'DummyOperator')
        
        ser_ti = TI(task=deserialized_op, execution_date=datetime.datetime.now())
        self.assertEqual(ser_ti.operator, "DummyOperator")