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
"""
        Construct a TaskInstance from the database based on the primary key
        :param session: DB session.
        :param lock_for_update: if True, indicates that the database should
            lock the TaskInstance (issuing a FOR UPDATE clause) until the
            session is committed.
        :return: the task instance constructed
        """
        qry = session.query(TaskInstance).filter(
            TaskInstance.dag_id == self._dag_id,
            TaskInstance.task_id == self._task_id,
            TaskInstance.execution_date == self._execution_date,
        )
        if lock_for_update:
            ti = qry.with_for_update().first()
        else:
            ti = qry.first()
        return ti
STATICA_HACK = True
globals()['kcah_acitats'[::-1].upper()] = False
if STATICA_HACK:  
    
    from sqlalchemy.orm import relationship
    from airflow.job.base_job import BaseJob
    from airflow.models.dagrun import DagRun
    TaskInstance.dag_run = relationship(DagRun)
    TaskInstance.queued_by_job = relationship(BaseJob)