**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.
airflow/api_connexion/endpoints/xcom_endpoint.py
query = query.join(DR, and_(XCom.dag_id == DR.dag_id, XCom.execution_date == DR.execution_date))
    if task_id != '~':
        query = query.filter(XCom.task_id == task_id)
    if dag_run_id != '~':
        query = query.filter(DR.run_id == dag_run_id)
    query = query.order_by(XCom.execution_date, XCom.task_id, XCom.dag_id, XCom.key)
    total_entries = query.count()
    query = query.offset(offset).limit(limit)
    return xcom_collection_schema.dump(XComCollection(xcom_entries=query.all(), total_entries=total_entries))
@security.requires_access(
    [
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG),
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_DAG_RUN),
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_TASK_INSTANCE),
        (permissions.ACTION_CAN_READ, permissions.RESOURCE_XCOM),
    ]
)
@provide_session
def get_xcom_entry(
    dag_id: str, task_id: str, dag_run_id: str, xcom_key: str, session: Session
) -> XComCollectionItemSchema:
    """Get an XCom entry"""
    query = session.query(XCom).filter(XCom.dag_id == dag_id, XCom.task_id == task_id, XCom.key == xcom_key)
    query = query.join(DR, and_(XCom.dag_id == DR.dag_id, XCom.execution_date == DR.execution_date))
    query = query.filter(DR.run_id == dag_run_id)
    query_object = query.one_or_none()
    if not query_object:
        raise NotFound("XCom entry not found")
    return xcom_collection_item_schema.dump(query_object)