Cancel date not set to today when left empty (CRM_Utils_System::isNull needs to handle 'null' string as a null value)

When cancelling an existing contribution, the Cancel Date is supposed to be set to TODAY if no value is selected from the date picker. This happens because the form passes 'null' string, but postProcess uses CRM_Utils_System::isNull to see if the form value is empty.

I've added a hack to isNull () to also check for 'null' string. We should remove that when we have DAO code that can pass null values to a DB column w/o passing 'null'.