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
for task_id, expected_state in ti_state_mapping.items():
            task_instance = dag_run.get_task_instance(task_id=task_id)
            self.assertEqual(task_instance.state, expected_state, error_message)
    @parameterized.expand(
        [
            (
                {('scheduler', 'schedule_after_task_execution'): 'True'},
                {'A': 'B', 'B': 'C'},
                {'A': State.QUEUED, 'B': State.NONE, 'C': State.NONE},
                {'A': State.SUCCESS, 'B': State.SCHEDULED, 'C': State.NONE},
                {'A': State.SUCCESS, 'B': State.SUCCESS, 'C': State.SCHEDULED},
                "A -> B -> C, with fast-follow ON when A runs, B should be QUEUED. Same for B and C.",
            ),
            (
                {('scheduler', 'schedule_after_task_execution'): 'False'},
                {'A': 'B', 'B': 'C'},
                {'A': State.QUEUED, 'B': State.NONE, 'C': State.NONE},
                {'A': State.SUCCESS, 'B': State.NONE, 'C': State.NONE},
                None,
                "A -> B -> C, with fast-follow OFF, when A runs, B shouldn't be QUEUED.",
            ),
            (
                {('scheduler', 'schedule_after_task_execution'): 'True'},
                {'A': 'B', 'C': 'B', 'D': 'C'},
                {'A': State.QUEUED, 'B': State.NONE, 'C': State.NONE, 'D': State.NONE},