DMNFTF — variable should be optional in InformationRequirement

According to the beta, the "variable" attribute is mandatory in the definition of an InformationRequirement.

However, it is useful only if the decisionLogic of the requesting Decision element is defined AND that decisionLogic uses the corresponding InformationItem.

It is not blocking to have to create and carry around the InfoItem anyway, but it is inconvenient. There is a risk that validating implementations will reject models where the unused variables are not defined.

Propose to make variable optional in InformationRequirement (so you do not have to create and interchange it, e.g. when no decision logic is specified that uses it)