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, state: Optional[str] = None, session: Session = None
    ) -> Optional[pendulum.DateTime]:
        """
        The start date from property previous_ti_success.
        :param state: If passed, it only take into account instances of a specific state.
        :param session: SQLAlchemy ORM Session
        """
        self.log.debug("previous_start_date was called")
        prev_ti = self.get_previous_ti(state=state, session=session)
        return prev_ti and prev_ti.start_date
    @property
    def previous_start_date_success(self) -> Optional[pendulum.DateTime]:
        """
        This attribute is deprecated.
        Please use `airflow.models.taskinstance.TaskInstance.get_previous_start_date` method.
        """
        warnings.warn(
            """
            This attribute is deprecated.
            Please use `airflow.models.taskinstance.TaskInstance.get_previous_start_date` method.
            """,
            DeprecationWarning,
            stacklevel=2,
        )
        return self.get_previous_start_date(state=State.SUCCESS)
    @provide_session
    def are_dependencies_met(self, dep_context=None, session=None, verbose=False):