I may not be understanding exactly what you're trying to do, so if I am off base, just ignore my response
Try putting in the criteria your combo box name and then underneath that in the next criteria (for the OR) put in your combo box name and Is Null:
[Forms]![frmVendors]![cboComponent] Is Null
That way if something is selected in the combo box, it should return that record. Otherwise, if nothing is selected it should return all records.
Hope this is what you're looking for!