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
"""
            Wrapper around Variable. This way you can get variables in
            templates by using ``{{ var.value.variable_name }}`` or
            ``{{ var.value.get('variable_name', 'fallback') }}``.
            """
            def __init__(self):
                self.var = None
            def __getattr__(
                self,
                item: str,
            ):
                self.var = Variable.get(item)
                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 value"""
                return Variable.get(item, default_var=default_var)
        class VariableJsonAccessor:
            """
            Wrapper around Variable. This way you can get variables in
            templates by using ``{{ var.json.variable_name }}`` or
            ``{{ var.json.get('variable_name', {'fall': 'back'}) }}``.
            """
            def __init__(self):