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
self.log.exception(e, exc_info=True)
                if registered:
                    
                    self._update_ti_state_for_sensing()
            
            with set_current_context(context):
                result = self._execute_task(context, task_copy)
            
            task_copy.post_execute(context=context, result=result)
        Stats.incr(f'operator_successes_{self.task.task_type}', 1, 1)
        Stats.incr('ti_successes')
    @provide_session
    def _update_ti_state_for_sensing(self, session=None):
        self.log.info('Submitting %s to sensor service', self)
        self.state = State.SENSING
        self.start_date = timezone.utcnow()
        session.merge(self)
        session.commit()
        
        raise AirflowSmartSensorException("Task successfully registered in smart sensor.")
    def _run_success_callback(self, context, task):
        """Functions that need to be run if Task is successful"""
        
        try:
            if task.on_success_callback:
                task.on_success_callback(context)
        except Exception as exc:  
            self.log.error("Failed when executing success callback")
            self.log.exception(exc)
    def _execute_task(self, context, task_copy):