declare
type emp_refcursor  is ref cursor;
emp_cv emp_refcursor;
emprec employees%rowtype;
stmt varchar2(100):= 'select * from employees';
   v_deptid number :=10;
begin
   if v_deptid is  null then open emp_cv for stmt;
else
    stmt :=stmt ||'where department_id := v_deptid';
end if;
    open emp_cv for stmt using v_deptid;
loop
fetch emp_cv into emprec;
dbms_output.put_line(emprec.salary);
end loop;
close emp_cv;
end;

SCOTT@orcl> declare
  2    type emp_refcursor is ref cursor;
  3    emp_cv	 emp_refcursor;
  4    emprec	 employees%rowtype;
  5    stmt	 varchar2(100) := 'select * from employees';
  6    v_deptid  number := 10;
  7  begin
  8    if v_deptid is null then
  9  	 open emp_cv for stmt;
 10    else
 11  	 -- need space before "where" and
 12  	 -- need "department_id = :b_deptid" instead of "department_id := v_deptid" to use bind variable:
 13  	 stmt := stmt || ' where department_id = :b_deptid';
 14  	 -- need to open modified stmt here, not after endif:
 15  	 open emp_cv for stmt using v_deptid;
 16    end if;
 17    loop
 18  	 fetch emp_cv into emprec;
 19  	 -- need exit command to avoid endless loop:
 20  	 exit when emp_cv%notfound;
 21  	 dbms_output.put_line (emprec.salary);
 22    end loop;
 23    close emp_cv;
 24  end;
 25  /
2450
5000
1300

PL/SQL procedure successfully completed.