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_pendulum_template_dates',
            schedule_interval='0 12 * * *',
            start_date=timezone.datetime(2016, 6, 1, 0, 0, 0),
        )
        task = DummyOperator(task_id='test_pendulum_template_dates_task', dag=dag)
        ti = TI(task=task, execution_date=timezone.utcnow())
        template_context = ti.get_template_context()
        self.assertIsInstance(template_context["execution_date"], pendulum.DateTime)
        self.assertIsInstance(template_context["next_execution_date"], pendulum.DateTime)
        self.assertIsInstance(template_context["prev_execution_date"], pendulum.DateTime)
    @parameterized.expand(
        [
            ('{{ var.value.a_variable }}', 'a test value'),
            ('{{ var.value.get("a_variable") }}', 'a test value'),
            ('{{ var.value.get("a_variable", "unused_fallback") }}', 'a test value'),
            ('{{ var.value.get("missing_variable", "fallback") }}', 'fallback'),
        ]
    )
    def test_template_with_variable(self, content, expected_output):