RadPivotGrid returns wrong results when a null-values-containing column is filtered.


Steps to reproduce:
1. Run the following code

2. Click the filter button on the second column and try to filter by "1" value

Result: the blank results only are shown.


<telerik:RadPivotGrid RenderMode="Lightweight" ID="TManagerGrid" runat="server" AllowFiltering="true"
            OnNeedDataSource="TManagerGrid_NeedDataSource"
            ShowFilterHeaderZone="true">
            <Fields>
                <telerik:PivotGridRowField DataField="NUM" UniqueName="NUM" ZoneIndex="10" SortOrder="Ascending">
                </telerik:PivotGridRowField>
                <telerik:PivotGridRowField DataField="SOURCE_LINE_ITEM" UniqueName="SOURCE_LINE_ITEM" ZoneIndex="11" SortOrder="Ascending">
                </telerik:PivotGridRowField>
                <telerik:PivotGridRowField DataField="SORTORDER" UniqueName="SORTORDER" ZoneIndex="12" SortOrder="Ascending">
                </telerik:PivotGridRowField>
                <telerik:PivotGridAggregateField DataField="NUM">
                </telerik:PivotGridAggregateField>
            </Fields>
        </telerik:RadPivotGrid>



    protected void TManagerGrid_NeedDataSource(object sender, Telerik.Web.UI.PivotGridNeedDataSourceEventArgs e)
    {
        TManagerGrid.DataSource = GetData();
    }

    protected DataTable GetData()
    {
        DataTable tbl = new DataTable();
        tbl.Columns.Add(new DataColumn("NUM"));
        tbl.Columns.Add(new DataColumn("SOURCE_LINE_ITEM"));
        tbl.Columns.Add(new DataColumn("SORTORDER"));

        tbl.Rows.Add(new object[] { "aa", 1, 20 });
        tbl.Rows.Add(new object[] { "bb", null, 30});
        tbl.Rows.Add(new object[] { "cc", 2, 50 });

        return tbl;
    }