**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
"offset=0",
                [
                    "TEST_XCOM_KEY1",
                    "TEST_XCOM_KEY10",
                    "TEST_XCOM_KEY2",
                    "TEST_XCOM_KEY3",
                    "TEST_XCOM_KEY4",
                    "TEST_XCOM_KEY5",
                    "TEST_XCOM_KEY6",
                    "TEST_XCOM_KEY7",
                    "TEST_XCOM_KEY8",
                    "TEST_XCOM_KEY9",
                ],
            ),
            (
                "limit=1&offset=5",
                ["TEST_XCOM_KEY5"],
            ),
            (
                "limit=1&offset=1",
                ["TEST_XCOM_KEY10"],
            ),
            (
                "limit=2&offset=2",
                ["TEST_XCOM_KEY2", "TEST_XCOM_KEY3"],
            ),
        ]
    )
    @provide_session
    def test_handle_limit_offset(self, query_params, expected_xcom_ids, session):