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
dag = models.DAG(dag_id='test_not_requeue_non_requeueable_task_instance')
        
        
        task = BaseSensorOperator(
            task_id='test_not_requeue_non_requeueable_task_instance_op',
            dag=dag,
            pool='test_pool',
            owner='airflow',
            start_date=timezone.datetime(2016, 2, 1, 0, 0, 0),
        )
        ti = TI(task=task, execution_date=timezone.utcnow(), state=State.QUEUED)
        with create_session() as session:
            session.add(ti)
            session.commit()
        all_deps = RUNNING_DEPS | task.deps
        all_non_requeueable_deps = all_deps - REQUEUEABLE_DEPS
        patch_dict = {}
        for dep in all_non_requeueable_deps:
            class_name = dep.__class__.__name__
            dep_patch = patch(f'{dep.__module__}.{class_name}.{dep._get_dep_statuses.__name__}')
            method_patch = dep_patch.start()
            method_patch.return_value = iter([TIDepStatus('mock_' + class_name, True, 'mock')])
            patch_dict[class_name] = (dep_patch, method_patch)
        for class_name, (dep_patch, method_patch) in patch_dict.items():
            method_patch.return_value = iter([TIDepStatus('mock_' + class_name, False, 'mock')])
            ti.run()
            self.assertEqual(ti.state, State.QUEUED)
            dep_patch.return_value = TIDepStatus('mock_' + class_name, True, 'mock')