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='op_no_dag')
        ti = TI(task=op_no_dag, execution_date=naive_datetime)
        self.assertEqual(ti.execution_date, DEFAULT_DATE)
        
        dag = DAG('dag', start_date=DEFAULT_DATE)
        op1 = DummyOperator(task_id='op_1')
        dag.add_task(op1)
        ti = TI(task=op1, execution_date=naive_datetime)
        self.assertEqual(ti.execution_date, DEFAULT_DATE)
        
        tzinfo = pendulum.timezone("Europe/Amsterdam")
        execution_date = timezone.datetime(2016, 1, 1, 1, 0, 0, tzinfo=tzinfo)
        utc_date = timezone.convert_to_utc(execution_date)
        ti = TI(task=op1, execution_date=execution_date)
        self.assertEqual(ti.execution_date, utc_date)
    def test_task_naive_datetime(self):