Java 转换，null字段丢失

[nn200433]

示例 json

{
    "currentVersion": {},
    "checkUpdate": {},
    "appInfo": {
        "copyright": "xxxxxxx",
        "mchId": null,
        "launchPageType": "0",
        "about": "<p>关于<\/p>",
        "type": "1",
        "homePageColStyle": 4,
        "navBarColStyle": 4,
        "appCenterColStyle": 4,
        "homePageRowStyle": 2,
        "launchPageFile": "/view/name/2aff40245ebc429ea1e57c1cd3f11e2c.jpg",
        "protocolName": "协议",
        "isShowLaunchPage": "1",
        "webUrl": null,
        "appId": "12",
        "name": "xxxApp",
        "alias": "xxxApp",
        "logo": "/view/name/55253353666649209447690781038e6f.png",
        "userType": "0",
        "protocolContent": [{
            "ext": "pdf",
            "size": 323669,
            "name": "隐私策略.pdf",
            "mbSize": "316.08 KB",
            "url": "6009028dd5cc4a548984ead5d9213be3.pdf"
        }]
    }
}