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.var = None
            def __getattr__(
                self,
                item: str,
            ):
                self.var = Variable.get(item, deserialize_json=True)
                return self.var
            def __repr__(self):
                return str(self.var)
            @staticmethod
            def get(
                item: str,
                
                default_var: Any = Variable._Variable__NO_DEFAULT_SENTINEL,  
            ):
                """Get Airflow Variable after deserializing JSON value"""
                return Variable.get(item, default_var=default_var, deserialize_json=True)
        return {
            'conf': conf,
            'dag': task.dag,
            'dag_run': dag_run,
            'ds': ds,
            'ds_nodash': ds_nodash,
            'execution_date': pendulum.instance(self.execution_date),
            'inlets': task.inlets,
            'macros': macros,
            'next_ds': next_ds,
            'next_ds_nodash': next_ds_nodash,
            'next_execution_date': next_execution_date,