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_id = 'test_generate_command_default_param'
        task_id = 'task'
        assert_command = ['airflow', 'tasks', 'run', dag_id, task_id, DEFAULT_DATE.isoformat()]
        generate_command = TI.generate_command(dag_id=dag_id, task_id=task_id, execution_date=DEFAULT_DATE)
        assert assert_command == generate_command
    def test_generate_command_specific_param(self):
        dag_id = 'test_generate_command_specific_param'
        task_id = 'task'
        assert_command = [
            'airflow',
            'tasks',
            'run',
            dag_id,
            task_id,
            DEFAULT_DATE.isoformat(),
            '--mark-success',
        ]
        generate_command = TI.generate_command(
            dag_id=dag_id, task_id=task_id, execution_date=DEFAULT_DATE, mark_success=True
        )
        assert assert_command == generate_command
    def test_get_rendered_template_fields(self):