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
task_b = PythonOperator(task_id='B', python_callable=python_callable)
                task_c = PythonOperator(task_id='C', python_callable=python_callable)
                if 'D' in init_state:
                    task_d = PythonOperator(task_id='D', python_callable=python_callable)
                for upstream, downstream in dependencies.items():
                    dag.set_dependency(upstream, downstream)
            scheduler = SchedulerJob()
            scheduler.dagbag.bag_dag(dag, root_dag=dag)
            dag_run = dag.create_dagrun(run_id='test_dagrun_fast_follow', state=State.RUNNING)
            task_instance_a = dag_run.get_task_instance(task_id=task_a.task_id)
            task_instance_a.task = task_a
            task_instance_a.set_state(init_state['A'])
            task_instance_b = dag_run.get_task_instance(task_id=task_b.task_id)
            task_instance_b.task = task_b
            task_instance_b.set_state(init_state['B'])
            task_instance_c = dag_run.get_task_instance(task_id=task_c.task_id)
            task_instance_c.task = task_c
            task_instance_c.set_state(init_state['C'])
            if 'D' in init_state:
                task_instance_d = dag_run.get_task_instance(task_id=task_d.task_id)
                task_instance_d.task = task_d
                task_instance_d.state = init_state['D']
            session.commit()
            task_instance_a.run()
            self.validate_ti_states(dag_run, first_run_state, error_message)
            if second_run_state:
                scheduler._critical_section_execute_task_instances(session=session)
                task_instance_b.run()
                self.validate_ti_states(dag_run, second_run_state, error_message)
    def test_set_state_up_for_retry(self):