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.error("Received SIGTERM. Terminating subprocesses.")
            task_copy.on_kill()
            raise AirflowException("Task received SIGTERM signal")
        signal.signal(signal.SIGTERM, signal_handler)
        
        self.clear_xcom_data()
        with Stats.timer(f'dag.{task_copy.dag_id}.{task_copy.task_id}.duration'):
            self.render_templates(context=context)
            RenderedTaskInstanceFields.write(RenderedTaskInstanceFields(ti=self, render_templates=False))
            RenderedTaskInstanceFields.delete_old_records(self.task_id, self.dag_id)
            
            airflow_context_vars = context_to_airflow_vars(context, in_env_var_format=True)
            self.log.info(
                "Exporting the following env vars:\n%s",
                '\n'.join([f"{k}={v}" for k, v in airflow_context_vars.items()]),
            )
            os.environ.update(airflow_context_vars)
            
            task_copy.pre_execute(context=context)
            
            self._run_execute_callback(context, task)
            if task_copy.is_smart_sensor_compatible():
                
                registered = False
                try:
                    registered = task_copy.register_in_sensor_service(self, context)
                except Exception as e:
                    self.log.warning(
                        "Failed to register in sensor service.Continue to run task in non smart sensor mode."
                    )