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
['one_success', 5, 0, 0, 0, 5, True, None, True],
            ['one_success', 2, 0, 0, 0, 2, True, None, True],
            ['one_success', 2, 0, 1, 0, 3, True, None, True],
            ['one_success', 2, 1, 0, 0, 3, True, None, True],
            #
            
            #
            ['all_failed', 5, 0, 0, 0, 5, True, State.SKIPPED, False],
            ['all_failed', 0, 0, 5, 0, 5, True, None, True],
            ['all_failed', 2, 0, 0, 0, 2, True, State.SKIPPED, False],
            ['all_failed', 2, 0, 1, 0, 3, True, State.SKIPPED, False],
            ['all_failed', 2, 1, 0, 0, 3, True, State.SKIPPED, False],
            #
            
            #
            ['one_failed', 5, 0, 0, 0, 0, True, None, False],
            ['one_failed', 2, 0, 0, 0, 0, True, None, False],
            ['one_failed', 2, 0, 1, 0, 0, True, None, True],
            ['one_failed', 2, 1, 0, 0, 3, True, None, False],
            ['one_failed', 2, 3, 0, 0, 5, True, State.SKIPPED, False],
            #
            
            #
            ['all_done', 5, 0, 0, 0, 5, True, None, True],
            ['all_done', 2, 0, 0, 0, 2, True, None, False],
            ['all_done', 2, 0, 1, 0, 3, True, None, False],
            ['all_done', 2, 1, 0, 0, 3, True, None, False],
        ]
    )
    def test_check_task_dependencies(