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
{
                        'args': [
                            'airflow',
                            'tasks',
                            'run',
                            'test_get_rendered_k8s_spec',
                            'op1',
                            '2016-01-01T00:00:00+00:00',
                        ],
                        'image': ':',
                        'name': 'base',
                        'env': [{'name': 'AIRFLOW_IS_K8S_EXECUTOR_POD', 'value': 'True'}],
                    }
                ]
            },
        }
        with create_session() as session:
            rtif = RenderedTaskInstanceFields(ti)
            session.add(rtif)
            self.assertEqual(rtif.k8s_pod_yaml, expected_pod_spec)
        
        with DAG('test_get_rendered_k8s_spec', start_date=DEFAULT_DATE):
            new_task = BashOperator(task_id='op1', bash_command="{{ task.task_id }}")
        new_ti = TI(task=new_task, execution_date=DEFAULT_DATE)
        pod_spec = new_ti.get_rendered_k8s_spec()
        self.assertEqual(expected_pod_spec, pod_spec)
        
        with create_session() as session:
            session.query(RenderedTaskInstanceFields).delete()
    def validate_ti_states(self, dag_run, ti_state_mapping, error_message):