**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
#
#
from typing import Optional
from flask import current_app, g
from sqlalchemy import and_
from sqlalchemy.orm.session import Session
from airflow.api_connexion import security
from airflow.api_connexion.exceptions import NotFound
from airflow.api_connexion.parameters import check_limit, format_parameters
from airflow.api_connexion.schemas.xcom_schema import (
    XComCollection,
    XComCollectionItemSchema,
    XComCollectionSchema,
    xcom_collection_item_schema,
    xcom_collection_schema,
)