🐛: 修复无所属用例步骤的bug

[JayWenStar]

删除控件时，当所属步骤无所属用例时，返回如下：

{
    "code": 2000,
    "message": "查询成功！",
    "data": [
        {
            "id": 4,
            "projectId": 1,
            "publicStepsId": null,
            "caseId": 1,
            "platform": 1,
            "stepType": "click",
            "content": "",
            "text": "",
            "sort": 4,
            "error": 3,
            "elements": null,
            "testCasesDTO": {
                "id": 1,
                "name": "B站用例1",
                "platform": 1,
                "projectId": 1,
                "module": "",
                "version": "",
                "des": "",
                "designer": "admin",
                "editTime": "2022-01-09 01:32:19"
            }
        },
        {
            "id": 14,
            "projectId": 1,
            "publicStepsId": null,
            "caseId": 2,
            "platform": 1,
            "stepType": "click",
            "content": "",
            "text": "",
            "sort": 14,
            "error": 3,
            "elements": null,
            "testCasesDTO": {
                "id": 2,
                "name": "B站用例2",
                "platform": 1,
                "projectId": 1,
                "module": "模块1",
                "version": "",
                "des": "",
                "designer": "admin",
                "editTime": "2022-01-09 02:14:34"
            }
        },
        {
            "id": 17,
            "projectId": 1,
            "publicStepsId": null,
            "caseId": 0,
            "platform": 1,
            "stepType": "click",
            "content": "",
            "text": "",
            "sort": 17,
            "error": 3,
            "elements": null,
            "testCasesDTO": {
                "id": 0,
                "name": "无所属用例",
                "platform": null,
                "projectId": null,
                "module": null,
                "version": null,
                "des": null,
                "designer": null,
                "editTime": null
            }
        }
    ]
}