ScheduleExpression.getTimezone() should return null for default @Schedule

For a default schedule when timezone is not explicitly specified, the EJB3.1 spec says:

[97] Note that annotation java.lang.String attributes use the empty string "" as a default, so the expression @Schedule(timezone="", ...) will result in a null value from the corresponding ScheduleExpression.getTimezone() method.

Right now, ScheduleExpression.getTimezone() returns an empty string for a default schedule:

ScheduleExpression exp = new ScheduleExpression().year(5000);
exp.getTimezone() // should return null;