Deals without StartDate or StartDateTime in ERN-3 and ERN 4.1

The StartDate or StartDateTime in the Deal composite was optional in ERN-3, ERN 4.0 and ERN 4.1; from ERN 4.2 onwards it will be mandatory. 

The reason for this change is that the semantics of having no StartDate or StartDateTime is ambiguous and DDEX strongly recommends to always communicate a StartDate or a StartDateTime in every Deal.