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
self,
        trigger_rule,
        successes,
        skipped,
        failed,
        upstream_failed,
        done,
        flag_upstream_failed,
        expect_state,
        expect_completed,
    ):
        start_date = timezone.datetime(2016, 2, 1, 0, 0, 0)
        dag = models.DAG('test-dag', start_date=start_date)
        downstream = DummyOperator(task_id='downstream', dag=dag, owner='airflow', trigger_rule=trigger_rule)
        for i in range(5):
            task = DummyOperator(task_id=f'runme_{i}', dag=dag, owner='airflow')
            task.set_downstream(downstream)
        run_date = task.start_date + datetime.timedelta(days=5)
        ti = TI(downstream, run_date)
        dep_results = TriggerRuleDep()._evaluate_trigger_rule(  
            ti=ti,
            successes=successes,
            skipped=skipped,
            failed=failed,
            upstream_failed=upstream_failed,