In the AddODataExpression method of the QueryableExtension.cs file, the null check for FilterValue is missing the IsEmpty and IsNotEmpty operators.
Radzen.Blazor/QueryableExtension.cs
if (filter.Property == null || filter.FilterOperator == null || (filter.FilterValue == null &&
filter.FilterOperator != FilterOperator.IsNull && filter.FilterOperator != FilterOperator.IsNotNull
&& filter.FilterOperator != FilterOperator.IsEmpty && filter.FilterOperator != FilterOperator.IsNotEmpty))
{
return;
}