如何验证给定节点的存在并从azure数据工厂读取值

我有一个查找,我正在执行查询以获取 employee_no

SELECT employee_no FROM HRS.EMPLOYEE_MASTER WHERE SOC_SEC_NO = '@{variables('FE_SoSecNo')}' 这可能会返回如下数据

{
    "firstRow": {
        "EMPLOYEE_NO": "12345"
    },
    "effectiveIntegrationRuntime": "VMSHIntegrationRuntime",
    "billingReference": {
        "activityType": "PipelineActivity",
        "billableDuration": [
            {
                "meterType": "SelfhostedIR",
                "duration": 0.016666666666666666,
                "unit": "Hours"
            }
        ],
        "totalBillableDuration": [
            {
                "meterType": "SelfhostedIR",
                "duration": 0.016666666666666666,
                "unit": "Hours"
            }
        ]
    },
    "durationInQueue": {
        "integrationRuntimeQueue": 3
    }
}

或者如果没有找到数据

{
    "effectiveIntegrationRuntime": "VMSHIntegrationRuntime",
    "billingReference": {
        "activityType": "PipelineActivity",
        "billableDuration": [
            {
                "meterType": "SelfhostedIR",
                "duration": 0.016666666666666666,
                "unit": "Hours"
            }
        ],
        "totalBillableDuration": [
            {
                "meterType": "SelfhostedIR",
                "duration": 0.016666666666666666,
                "unit": "Hours"
            }
        ]
    },
    "durationInQueue": {
        "integrationRuntimeQueue": 3
    }
}

我添加了一个步骤来读取值,如下所示,但当json节点不存在时,如果节点值不存在,我将尝试插入什么,并通过获取值来更新

activity('GetEmpBySSN').output.firstRow.employee_no

我能知道如何写逻辑来读取值吗