下拉Kendo Angular UI的默认值和空值

Question

这个用于Kendo Angular下拉列表的HTML为空/默认值留下一个逗号，并且在我选择了一个值后不允许我返回空值。我在我的模板上玩过各种各样的三元运算符无济于事。我是否应该为 LastName，FirstName 的所有实例创建一个管道以使事情变得更容易？

 <kendo-dropdownlist id="cost-engineer-dropdown"
    formControlName="CostEngineerId"
    [defaultItem]="{ text: 'Select an Engineer', value: '' }"
    [data]="costEngineer | async"
    [textField]="'LastName'"
    [valueField]="'Id'"
    value="this.LastName"
    (valueChange)="valueChange('this.Id')"
    (selectionChange)="selectionChange($event)"
    [formControl]="masterProjectFormGroup.get('CostEngineerId')"
    [valuePrimitive]="true"
    [(ngModel)]="CostEngineerId"
    >
<ng-template kendoDropDownListItemTemplate let-dataItem>
    {{dataItem?.LastName}}, {{dataItem?.FirstName}}
</ng-template>
<ng-template kendoDropDownListValueTemplate let-dataItem>
    {{dataItem?.LastName}}, {{dataItem?.FirstName}}
</ng-template>
</kendo-dropdownlist>