FTSearch is meant for full-text search of all collected documents. You are trying to search the documents whose employeeID field value is null, but FT Search not work if you want to search for the specific field value…You should specify a string for FTSearch and it looks into all the documents whether any field value (not particular field) contains that text and it collects that document if any field contain the string value.
For your case you have to use either Getallentriesbykey or getalldocumentsbykey …remember the view should be categories or sorted