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
#
#
#
import datetime
import os
import time
import unittest
import urllib
from typing import List, Optional, Union, cast
from unittest import mock
from unittest.mock import call, mock_open, patch
import pendulum
import pytest
from freezegun import freeze_time
from parameterized import param, parameterized
from sqlalchemy.orm.session import Session
from airflow import models, settings
from airflow.exceptions import AirflowException, AirflowFailException, AirflowSkipException
from airflow.jobs.scheduler_job import SchedulerJob
from airflow.models import (