MuleSoft DataWeave检查XML文件是否存在

我有以下mule DataWeave转换：

([]) when (payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments =="") otherwise 
{
Id: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.@id as :string,
Date: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Date,
Time: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Time,
Cancel: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Cancel as :string,
VisitType: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#VisitType,
VisitTypeID: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#VisitTypeID as :string,
Duration: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Duration as :string,
Confirm: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Confirm as :string,
Providers: {
    Provider: {
        Id: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Providers.ns0#Provider.@id as :string,
        Name: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Providers.ns0#Provider.@name,
        Department: {
            Id: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Providers.ns0#Provider.ns0#Department.@id as :string,
            Name: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Providers.ns0#Provider.ns0#Department.@name,
            Center: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Providers.ns0#Provider.ns0#Department.@center,
            DepartmentDirections: "" when payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Providers.ns0#Provider.ns0#Department.ns0#DepartmentDirections == null otherwise payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Providers.ns0#Provider.ns0#Department.ns0#DepartmentDirections
        }
    }
},
PatientInstructions: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#PatientInstructions,
Copay: payload.ns0#GetFutureAppointmentsResponse.ns0#Appointments.ns0#Appointment.ns0#Copay as :string
}

除了字段DepartmentDirections不总是在XML中传递之外，其他一切都正常。所以现在，当我添加这个字段时，我的Web服务不会返回任何内容，但当我删除它时，它会工作。由于字段可能在那里，也可能不在那里，我认为在字段不在那里的情况下检查null不会有帮助。是否有一种方法可以轻松检查实际字段是否存在，而不是赋值前的值？

添加标题skipNullOn=everywhere，如果输入中不存在，则转换时将跳过字段。 %输出应用程序/json skipNullOn=everywhere 仅适用于XML和JSON

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仅供参考：

与您的问题没有直接关系，但请帮自己一个忙，并将其中的大部分提取到有效负载的%var中。ns0GetFutureAppointsResponse.ns0Appoints.ns0Appointment块。