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


airflow/models/taskinstance.py
return TaskInstance(task=self.task, execution_date=previous_scheduled_date)
            dr.dag = dag
            
            
            
            ignore_schedule = state is not None or dag.schedule_interval is None
            if dag.catchup is True and not ignore_schedule:
                last_dagrun = dr.get_previous_scheduled_dagrun(session=session)
            else:
                last_dagrun = dr.get_previous_dagrun(session=session, state=state)
            if last_dagrun:
                return last_dagrun.get_task_instance(self.task_id, session=session)
        return None
    @property
    def previous_ti(self):
        """
        This attribute is deprecated.
        Please use `airflow.models.taskinstance.TaskInstance.get_previous_ti` method.
        """
        warnings.warn(
            """
            This attribute is deprecated.
            Please use `airflow.models.taskinstance.TaskInstance.get_previous_ti` method.
            """,
            DeprecationWarning,
            stacklevel=2,
        )
        return self.get_previous_ti()
    @property
    def previous_ti_success(self) -> Optional['TaskInstance']: