Flow control in a user-defined function

I have created a function that responds to the following code:

  -- Pasando un numero de departamento, obtener los empleados de este y su id.

    CREATE FUNCTION F_Emp_NumEmp
    (@NumDe NUMERIC(3))
    RETURNS TABLE
    AS
        RETURN (
            SELECT
                td.NumDe, NumEm, Nombre
            FROM
                TDepto td INNER JOIN Temple tem ON (td.NumDe = tem.NumDe)
            WHERE
                td.NumDe = @NumDe
        );


    -- Invocacion
    SELECT 
        * 
    FROM
        dbo.F_Emp_NumEmp(121)

The problem I have is that I do not find any method to perform flow control within it, and I have not found any reference, I mean, what I'm trying to do is that when invoking this function, the introduced parameter does not return any value , show a personalized message, as well as if the same function is not sent any parameter, show another different message.