**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
(permissions.ACTION_CAN_READ, permissions.RESOURCE_XCOM),
            ],
        )
        cls.app.appbuilder.sm.sync_perm_for_dag(  
            "test-dag-id-1",
            access_control={'TestGranularDag': [permissions.ACTION_CAN_EDIT, permissions.ACTION_CAN_READ]},
        )
        create_user(cls.app, username="test_no_permissions", role_name="TestNoPermissions")  
    @classmethod
    def tearDownClass(cls) -> None:
        delete_user(cls.app, username="test")  
        delete_user(cls.app, username="test_no_permissions")  
    @staticmethod
    def clean_db():
        clear_db_dags()
        clear_db_runs()
        clear_db_xcom()
    def setUp(self) -> None:
        """
        Setup For XCom endpoint TC
        """
        self.client = self.app.test_client()  
        
        self.clean_db()
    def tearDown(self) -> None:
        """
        Clear Hanging XComs
        """
        self.clean_db()
class TestGetXComEntry(TestXComEndpoint):