Default to DataTypes::STRING when no constraints are found

If a property doesn't contain any validation constraints, the parser returns a parameter definition which only contain `null` values. This normally would fly during merging of parameters, but sometimes cause problems when the property names will differ, i.e., when `JMSMetadataParser` uses snake-cased parameters.

All this patch does is ensure that parameters returned by `ValidationParser` has an `actualType` property set.