select 组件在 combo 内 指定 labelField 和 valueField 时，第二个下拉框为空

#### 描述问题：

select 组件在 combo 内 指定 labelField 和 valueField 时，第二个下拉框为空

#### 截图或视频：
正常场景：
异常场景：

#### 如何复现（请务必完整填写下面内容）：
1. 你是如何使用 amis 的？
jssdk

2. amis 版本是什么？请先在最新 beta 版本测试问题是否存在
v3.0.0

3. 粘贴有问题的完整 `amis schema` 代码：
**正常配置**
- schema
```
{
"name": "select_combo",
"label": "下拉框demo",
"items": [
{
"name": "select",
"label": "下拉框",
"unique": true,
"source": {
"url": "/api/data"
},
"labelField": "label",
"valueField": "value",
"type": "select"
}
],
"multiple": true,
"type": "combo"
}
```
- /api/data
```
{
"code": 0,
"message": "操作成功",
"data": [
{
"value": "a",
"label": "A"
},
{
"value": "b",
"label": "B"
}
]
}
```

**异常配置**

- schema
```
{
"name": "select_combo",
"label": "下拉框demo",
"items": [
{
"name": "select",
"label": "下拉框",
"unique": true,
"source": {
"url": "/api/data"
},
"labelField": "label1",
"valueField": "value1",
"type": "select"
}
],
"multiple": true,
"type": "combo"
}
```
- /api/data
```
{
"code": 0,
"message": "操作成功",
"data": [
{
"value1": "a",
"label1": "A"
},
{
"value1": "b",
"label1": "B"
}
]
}
```

4. 操作步骤
请简单描述一下复现的操作步骤...

Start with the select component inside the combo schema shown in the report, using the JS SDK reproduction and its /api/data response as the entry point. Compare the normal label/value configuration with the label1/value1 configuration, then verify that the second dropdown is populated with the returned options and that the existing configuration still works.

Written by the indexing model from the issue text.