**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
XCom.set(
            key=xcom_key,
            value="TEST_VALUE",
            execution_date=execution_date,
            task_id=task_id,
            dag_id=dag_id,
        )
        dagrun = DR(
            dag_id=dag_id,
            run_id=dag_run_id,
            execution_date=execution_date,
            start_date=execution_date,
            run_type=DagRunType.MANUAL,
        )
        session.add(dagrun)
class TestGetXComEntries(TestXComEndpoint):
    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'
        execution_date_parsed = parse_execution_date(execution_date)
        dag_run_id = DR.generate_run_id(DagRunType.MANUAL, execution_date_parsed)
        self._create_xcom_entries(dag_id, dag_run_id, execution_date_parsed, task_id)
        response = self.client.get(