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


tests/models/test_taskinstance.py
dag.create_dagrun(
            execution_date=run_date,
            state=State.RUNNING,
            run_type=DagRunType.SCHEDULED,
        )
        ti = TI(task, run_date)
        
        task.run(start_date=run_date, end_date=run_date, ignore_first_depends_on_past=False)
        ti.refresh_from_db()
        self.assertIs(ti.state, None)
        
        task.run(start_date=run_date, end_date=run_date, ignore_first_depends_on_past=True)
        ti.refresh_from_db()
        self.assertEqual(ti.state, State.SUCCESS)
    
    
    
    
    @parameterized.expand(
        [
            #
            
            #
            ['all_success', 5, 0, 0, 0, 0, True, None, True],
            ['all_success', 2, 0, 0, 0, 0, True, None, False],
            ['all_success', 2, 0, 1, 0, 0, True, State.UPSTREAM_FAILED, False],
            ['all_success', 2, 1, 0, 0, 0, True, State.SKIPPED, False],
            #
            
            #