empty string for nullable LinqToSql fileds, when RecordField.Type not string

[people]

I have a JsonReader field declaration

 <ext:RecordField Name="DATE_ORDER" Type="Date"/>

declared as

public System.Nullable<System.DateTime> DATE_ORDER

in code behind file.

I want to show empty string for null value in GridPanel. How can I do that?

[people]

I found my problem with Date column. It's works fine, but I steel need solution with 'float' type.

 
 public System.Nullable<decimal> INVOICE_SUM{}

 <ext:RecordField Name="INVOICE_SUM" Type="Float"  SubmitEmptyValue="Null" />

 <ext:Column  Header="Invoice" DataIndex= "INVOICE_SUM" >

         <Renderer Fn="Ext.util.Format.numberRenderer('0,000.00')" />
         <Editor>
                <ext:TextField runat="server" />
          </Editor>
 </ext:Column>

For DBNull value in DB I see '0.00' in the Invoice DataPanel cell. When I submit some changes to DB INVOICE_SUM becomes '0'.
But if I clear '0.00' to empty string on editable "Invoice" column it works the way I need. So my question is, how to show string empty instead '0.00' for null value in float column.
I use LinqToSql generated entity class, asp:LinqDataSource and JsonReader.