Proper handling of Polymorphic types when JsonTypeInfo.Id is DEDUCTION

[jcrayner]

Schema generation for polymorphic types where the deserialisation strategy is of JsonTypeInfo.Id.DEDUCTION. Jackson uses a seralization strategy of JsonTypeInfo.Id.EXISTING_PROPERTY with a visibility of false and a null property name when writing polymorphic types such as this. The existing implementation relies on a property name being non-null with it's polymorphic type handling (when writing out the @type or @Class enum within the subtype), which is not appropriate for this particular strategy.