Modifying my AdvanceFilters class to the following gets the results I need.
public class MyAdvancedFilters : AdvancedFilters
{
/// <summary>
/// Initializes a new instance of the <see cref="MyAdvancedFilters"/> class.
/// </summary>
/// <param name="principal">The source <see cref="Principal"/></param>
public MyAdvancedFilters(Principal principal) : base(principal) { }
public void WhereCompanyNotSet()
{
this.AdvancedFilterSet("company", "*", typeof(string), MatchType.NotEquals);
}
}
It is the "*" for the value part in the AdvancedFilterSet.
Thanks to Sean for leading to the right trail to come up with the answer appropriate for the AccountManagement objects.