Closing a period should check for draft statement in the period
Currently, when a Period is closed, there is no check for potential existing statements which will be in the period. Such check already exists for assets for example.
Once the Period is closed, Statement will error out on validation due to closed Period (the move couldn't be created).
As the date is usually defined externally, it is a problem to have to change it because a period is locked (as with account_fr module)