postgres: dynamic conditional in function query

I have a Postgres function where I need to dynamically add a conditional parameter if it was passed into the function. Here's the function:

CREATE OR REPLACE FUNCTION public.get_appointments(
     for_business_id INTEGER,
     range_start DATE,
     range_end   DATE,
     for_staff_id INTEGER
    )
       RETURNS SETOF appointment
       LANGUAGE plpgsql STABLE
       AS $function$
     DECLARE
        appointment appointment;
        recurrence  TIMESTAMP;
        appointment_length INTERVAL;
        parent_id UUID;
     BEGIN
        FOR appointment IN
            -- NEED TO CONDITIONALLY ADD "WHERE staff_id = for_staff_id" 
            -- if for_staff_id is NOT NULL
            SELECT *
              FROM appointment
             WHERE business_id = for_business_id
               AND (
                       recurrence_pattern IS NOT NULL
                   OR  (
                          recurrence_pattern IS NULL
                      AND starts_at BETWEEN range_start AND range_end
                   )
               )
         LOOP
            IF appointment.recurrence_pattern IS NULL THEN
              RETURN NEXT appointment;
              CONTINUE;
            END IF;

            appointment_length := appointment.ends_at - appointment.starts_at;
            parent_id := appointment.id;

            FOR recurrence IN
                SELECT *
                  FROM generate_recurrences(
                           appointment.recurrence_pattern,
                           appointment.starts_at,
                           range_start,
                           range_end
                  )
            LOOP
                EXIT WHEN recurrence::date > range_end;
                CONTINUE WHEN (recurrence::date < range_start AND recurrence::date > range_end) OR recurrence = ANY(appointment.recurrence_exceptions);
                appointment.id := uuid_generate_v5(uuid_nil(), parent_id::varchar || recurrence::varchar);
                appointment.parent_id := parent_id;
                appointment.starts_at := recurrence;
                appointment.ends_at := recurrence + appointment_length;
                appointment.recurrence_pattern := appointment.recurrence_pattern;
                appointment.recurrence_exceptions := NULL;
                appointment.is_recurrence := true;
                RETURN NEXT appointment;
            END LOOP;
        END LOOP;
        RETURN;
     END;
    $function$;

If for_staff_id is passed into the function, I need to add it as a conditional (WHERE staff_id = for_staff_id) to the query that's located in the FOR...IN loop. I've tried doing an IF/ELSE within the FOR...IN, but I'm getting a syntax error.

Is there a better way to do this?

Thanks!

Note that I'm running Postgres 9.5.