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.create_dagrun(
                run_type=DagRunType.SCHEDULED,
                state=state,
                execution_date=execution_date,
                start_date=pendulum.now('UTC'),
                session=session,
            )
            ti = TI(task=task, execution_date=execution_date)
            ti.set_state(state=State.SUCCESS, session=session)
            return ti
        with create_session() as session:  
            date = cast(pendulum.DateTime, pendulum.parse('2019-01-01T00:00:00+00:00'))
            ret = []
            for idx, state in enumerate(scenario):
                new_date = date.add(days=idx)
                ti = get_test_ti(session, new_date, state)
                ret.append(ti)
            return ret
    _prev_dates_param_list = (
        param('cron/catchup', '0 0 * * * ', True),
        param('cron/no-catchup', '0 0 * * *', False),
        param('no-sched/catchup', None, True),
        param('no-sched/no-catchup', None, False),
        param('timedelta/catchup', datetime.timedelta(days=1), True),
        param('timedelta/no-catchup', datetime.timedelta(days=1), False),
    )
    @parameterized.expand(_prev_dates_param_list)
    def test_previous_ti(self, _, schedule_interval, catchup) -> None: