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 Number of running TIs from the DB"""
        
        return (
            session.query(func.count())
            .filter(
                TaskInstance.dag_id == self.dag_id,
                TaskInstance.task_id == self.task_id,
                TaskInstance.state == State.RUNNING,
            )
            .scalar()
        )
    def init_run_context(self, raw=False):
        """Sets the log context."""
        self.raw = raw
        self._set_context(self)
    @staticmethod
    def filter_for_tis(tis: Iterable[Union["TaskInstance", TaskInstanceKey]]) -> Optional[BooleanClauseList]:
        """Returns SQLAlchemy filter to query selected task instances"""
        if not tis:
            return None
        
        first = list(tis)[0]
        dag_id = first.dag_id
        execution_date = first.execution_date