ObjRelationship.isSourceIndependentFromtTarget() is not enough to detect optional relatonship

[Andrus Adamchik]

Periodically running into bugs with optional to-one relationships when  
people forget to check to-dep-pk checkbox, and end up having  
FaultFailureExceptions. The issue is as old as Cayenne (see for  
instance CAY-132). They way I think we should fix it is by introducing  
another method "ObjRelationship.isOptional()" which is logically not  
the same as 'isSourceIndependentFromtTarget()', for use inside  
DataDomainQueryAction.

I figured I'd post a note about this, considering how old the issue is.
Andrus