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
"""
        Returns the DagRun for this TaskInstance
        :param session: SQLAlchemy ORM Session
        :return: DagRun
        """
        from airflow.models.dagrun import DagRun  
        dr = (
            session.query(DagRun)
            .filter(DagRun.dag_id == self.dag_id, DagRun.execution_date == self.execution_date)
            .first()
        )
        return dr
    @provide_session
    def check_and_change_state_before_execution(  
        self,
        verbose: bool = True,
        ignore_all_deps: bool = False,
        ignore_depends_on_past: bool = False,
        ignore_task_deps: bool = False,
        ignore_ti_state: bool = False,
        mark_success: bool = False,
        test_mode: bool = False,
        job_id: Optional[str] = None,
        pool: Optional[str] = None,