DMN17 — No default value for InformationItem

Currently, the variables associated with InputDatas have both a name and a type reference. Adding an initializer/default value would have the following benefits:

• simplify the logic required when checking the values provided by users
• consistent with existing default values for Output Clauses in Decision Tables
• align with current FEEL functions with default values
• align with current approach in modern PLs & DSLs (e.g. Python)

Add @defaultValue to InformationItem

Add a defaultValue attribute in InformationItem and define the semantics of assignment to the associated variables. Since it is touching the same paragraph this includes a one-sentence fix for DMN17-173.