**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
def test_should_respond_200(self):
        dag_id = 'test-dag-id'
        task_id = 'test-task-id'
        execution_date = '2005-04-02T00:00:00+00:00'
        xcom_key = 'test-xcom-key'
        execution_date_parsed = parse_execution_date(execution_date)
        dag_run_id = DR.generate_run_id(DagRunType.MANUAL, execution_date_parsed)
        self._create_xcom_entry(dag_id, dag_run_id, execution_date_parsed, task_id, xcom_key)
        response = self.client.get(
            f"/api/v1/dags/{dag_id}/dagRuns/{dag_run_id}/taskInstances/{task_id}/xcomEntries/{xcom_key}",
            environ_overrides={'REMOTE_USER': "test"},
        )
        self.assertEqual(200, response.status_code)
        current_data = response.json
        current_data['timestamp'] = 'TIMESTAMP'
        self.assertEqual(
            current_data,
            {
                'dag_id': dag_id,
                'execution_date': execution_date,
                'key': xcom_key,
                'task_id': task_id,
                'timestamp': 'TIMESTAMP',
            },
        )
    def test_should_raises_401_unauthenticated(self):