Add user: no defaultcustomer if no customer is selected

problem:

• add user: when no customer is selected ^*), the new user gets the defaultcustomer assigned

fix:

• add user: don't assign defaultcustomer when no customer is selected. ^*)

^*) "selected" in the "Add user"-window, not in the CustomerSwitch. The CustomerSwitch does not preselect clients in the "Add-user"-window.