Bug: DataGrid filtering with "CheckBoxList with OData" on null navigation properties not working

[agateeno]

Describe the bug
Filtering a column with CheckboxList using OData with a nullable navigation property causes error, crashing the application

To Reproduce
Using the "CheckboxList with OData" example, edit the source:

• creating a new column in line 19
  <RadzenDataGridColumn Property="Employee.Employee1.LastName" Title="Employee1Demo" />
• edit the line 39
  var result = await service.GetOrders(filter: args.Filter, top: args.Top, skip: args.Skip, orderby: args.OrderBy, count: true, expand: "Customer,Employee($expand=Employee1)");

Expected behavior
Just like Excel, if there are null values, the first filter value should be "(null)", so the end user could get all rows with null

Screenshots