**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
"""
        Invalid XCom entries to test join query
        """
        for i in [1, 2]:
            XCom.set(
                key=f'invalid-xcom-key-{i}',
                value="TEST",
                execution_date=execution_date,
                task_id="invalid_task",
                dag_id="invalid_dag",
            )
        dag = DagModel(dag_id="invalid_dag")
        session.add(dag)
        dagrun = DR(
            dag_id="invalid_dag",
            run_id="invalid_run_id",
            execution_date=execution_date + timedelta(days=1),
            start_date=execution_date,
            run_type=DagRunType.MANUAL,
        )
        session.add(dagrun)
        dagrun = DR(
            dag_id="invalid_dag_1",
            run_id="invalid_run_id",
            execution_date=execution_date,
            start_date=execution_date,
            run_type=DagRunType.MANUAL,
        )
        session.commit()
class TestPaginationGetXComEntries(TestXComEndpoint):