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.dag_id,
            self.task_id,
            self._safe_date('execution_date', '%Y%m%dT%H%M%S'),
            self._safe_date('start_date', '%Y%m%dT%H%M%S'),
            self._safe_date('end_date', '%Y%m%dT%H%M%S'),
        )
        if email_for_state and task.email:
            try:
                self.email_alert(error)
            except Exception as exec2:  
                self.log.error('Failed to send email to: %s', task.email)
                self.log.exception(exec2)
        
        if callback:
            try:
                callback(context)
            except Exception as exec3:  
                self.log.error("Failed at executing callback")
                self.log.exception(exec3)
        if not test_mode:
            session.merge(self)
        session.commit()
    def is_eligible_to_retry(self):
        """Is task instance is eligible for retry"""
        return self.task.retries and self.try_number <= self.max_tries
    def _safe_date(self, date_attr, fmt):
        result = getattr(self, date_attr, None)
        if result is not None:
            return result.strftime(fmt)
        return ''