**Apache Airflow version**: 2.0.0

**What happened**:

Using endpoint `/dags/{dag_id}/dagRuns/{dag_run_id}/taskInstances/{task_id}/xcomEntries/{xcom_key}` I got Response Body but without `value` entry. Like:
```
{
  "dag_id": "string",
  "execution_date": "string",
  "key": "string",
  "task_id": "string",
  "timestamp": "string"
}
```
Instead of:
```
{
  "dag_id": "string",
  "execution_date": "string",
  "key": "string",
  "task_id": "string",
  "timestamp": "string",
  "value": "string"
}
```
The exact value by defined `key` exists.
tests/api_connexion/endpoints/test_xcom_endpoint.py
f"/api/v1/dags/{dag_id}/dagRuns/{dag_run_id}/taskInstances/{task_id}/xcomEntries",
            environ_overrides={'REMOTE_USER': "test"},
        )
        self.assertEqual(200, response.status_code)
        response_data = response.json
        for xcom_entry in response_data['xcom_entries']:
            xcom_entry['timestamp'] = "TIMESTAMP"
        self.assertEqual(
            response.json,
            {
                'xcom_entries': [
                    {
                        'dag_id': dag_id,
                        'execution_date': execution_date,
                        'key': 'test-xcom-key-1',
                        'task_id': task_id,
                        'timestamp': "TIMESTAMP",
                    },
                    {
                        'dag_id': dag_id,
                        'execution_date': execution_date,
                        'key': 'test-xcom-key-2',
                        'task_id': task_id,
                        'timestamp': "TIMESTAMP",
                    },
                ],
                'total_entries': 2,
            },
        )
    def test_should_respond_200_with_tilde_and_access_to_all_dags(self):