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
self._clean()
    def tearDown(self) -> None:
        self._clean()
    @parameterized.expand(
        [
            
            (10, False),
            (5, True),
        ]
    )
    def test_execute_queries_count(self, expected_query_count, mark_success):
        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(expected_query_count):
            ti._run_raw_task(mark_success=mark_success)
    def test_execute_queries_count_store_serialized(self):