**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
dag_id_1 = 'test-dag-id-1'
        task_id_1 = 'test-task-id-1'
        execution_date = '2005-04-02T00:00:00+00:00'
        execution_date_parsed = parse_execution_date(execution_date)
        dag_run_id_1 = DR.generate_run_id(DagRunType.MANUAL, execution_date_parsed)
        self._create_xcom_entries(dag_id_1, dag_run_id_1, execution_date_parsed, task_id_1)
        dag_id_2 = 'test-dag-id-2'
        task_id_2 = 'test-task-id-2'
        dag_run_id_2 = DR.generate_run_id(DagRunType.MANUAL, execution_date_parsed)
        self._create_xcom_entries(dag_id_2, dag_run_id_2, execution_date_parsed, task_id_2)
        self._create_invalid_xcom_entries(execution_date_parsed)
        response = self.client.get(
            "/api/v1/dags/~/dagRuns/~/taskInstances/~/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_1,
                        'execution_date': execution_date,