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
"""
        Test the availability of variables in templates
        """
        Variable.set('a_variable', 'a test value')
        with DAG('test-dag', start_date=DEFAULT_DATE):
            task = DummyOperator(task_id='op1')
        ti = TI(task=task, execution_date=DEFAULT_DATE)
        context = ti.get_template_context()
        result = task.render_template(content, context)
        self.assertEqual(result, expected_output)
    def test_template_with_variable_missing(self):
        """
        Test the availability of variables in templates
        """
        with DAG('test-dag', start_date=DEFAULT_DATE):
            task = DummyOperator(task_id='op1')
        ti = TI(task=task, execution_date=DEFAULT_DATE)
        context = ti.get_template_context()
        with self.assertRaises(KeyError):
            task.render_template('{{ var.value.get("missing_variable") }}', context)
    @parameterized.expand(
        [
            ('{{ var.value.a_variable }}', '{\n  "a": {\n    "test": "value"\n  }\n}'),
            ('{{ var.json.a_variable["a"]["test"] }}', 'value'),
            ('{{ var.json.get("a_variable")["a"]["test"] }}', 'value'),
            ('{{ var.json.get("a_variable", {"a": {"test": "unused_fallback"}})["a"]["test"] }}', 'value'),
            ('{{ var.json.get("missing_variable", {"a": {"test": "fallback"}})["a"]["test"] }}', 'fallback'),
        ]
    )
    def test_template_with_json_variable(self, content, expected_output):