dropdown using Ajax

Hi
i have one drop down,suppose if drop down value should empty i want pass o or null value...if didnt pass anythg post method wont called
Eg
$.ajax({ type: 'POST', contentType: "application/json; charset=utf-8", url: 'EmployeeMaster.aspx/InsertMethod',
data:"{ 'Dept':'" + $('#drp_Department').val() + "'}"--Here how m going to pass o or null value when when getting dropdown value is empty


Thanks