**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
for i in [1, 2]:
            XCom.set(
                key=f'test-xcom-key-{i}',
                value="TEST",
                execution_date=execution_date,
                task_id=task_id,
                dag_id=dag_id,
            )
        dag = DagModel(dag_id=dag_id)
        session.add(dag)
        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)
    @provide_session
    def _create_invalid_xcom_entries(self, execution_date, session=None):