Ext.net RecordField SubmitEmptyValue invalid 无效问题有解决方案吗

<ext:Store 
            ID="Store1" 
            runat="server" 
            AutoSave="true" 
            ShowWarningOnFailure="false"
            OnBeforeStoreChanged="HandleChanges" 
            SkipIdForNewRecords="false"
            RefreshAfterSaving="None">
            <Reader>
                <ext:JsonReader IDProperty="Id">
                    <Fields>
                        <ext:RecordField Name="Id" />
                        <ext:RecordField Name="Email" AllowBlank="false" />
                        <ext:RecordField Name="First" AllowBlank="true" SubmitEmptyValue="Null" />
                        <ext:RecordField Name="Last" AllowBlank="true" SubmitEmptyValue="Undefined" />
                    </Fields>
                </ext:JsonReader>
            </Reader>
            
            <Listeners>
                <Exception Handler="
                    Ext.net.Notification.show({
                        iconCls    : 'icon-exclamation', 
                        html       : e.message, 
                        title      : 'EXCEPTION', 
                        autoScroll : true, 
                        hideDelay  : 5000, 
                        width      : 300, 
                        height     : 200
                    });" />
                <BeforeSave Handler="var valid = true; this.each(function(r){if(r.dirty && !r.isValid()){valid=false;}}); return valid;" />
            </Listeners>
        </ext:Store>

  <ext:FormPanel 

            ID="UserForm" 
            runat="server"
            Icon="User"
            Frame="true"
            LabelAlign="Right"
            Title="User -- All fields are required"
            Width="500">
            <Items>
                <ext:TextField runat="server"
                    FieldLabel="Email"
                    DataIndex="Email"
                    AllowBlank="false"
                    Vtype="email"
                    AnchorHorizontal="100%"
                    />
                
                <ext:TextField runat="server"
                    FieldLabel="First"
                    DataIndex="First"
                    AllowBlank="true"
                    AnchorHorizontal="100%"
                    />
                
                <ext:TextField runat="server"
                    FieldLabel="Last"
                    DataIndex="Last"
                    AllowBlank="true"
                    AnchorHorizontal="100%"
                    />
            </Items>
            
            <Buttons>
                <ext:Button 
                    runat="server"
                    Text="Save"
                    Icon="Disk">
                    <Listeners>
                        <Click Handler="updateRecord(#{UserForm});" />
                    </Listeners>
                </ext:Button>
                
                <ext:Button 
                    runat="server"
                    Text="Create"
                    Icon="UserAdd">
                    <Listeners>
                        <Click Handler="addRecord(#{UserForm}, #{GridPanel1});" />
                    </Listeners>
                </ext:Button>
                
                <ext:Button 
                    runat="server"
                    Text="Reset">
                    <Listeners>
                        <Click Handler="#{UserForm}.getForm().reset();" />
                    </Listeners>
                </ext:Button>
            </Buttons>
        </ext:FormPanel>

---

 我将

SubmitEmptyValue="Null" 和”Undefined“ 都试过了，当输入空值，也就是TextBox空白的情况后台对应的 

First，Last字段的值是StringEmpty，并不是NULL，这让我很头疼，那位大侠做过类似的项目，不知道是否有解决方案。