Nullable "anyOf" type support

[tadam313]

Motivation of this change

Nullable types in json schema often represented as "field":{ "anyOf":[{"type": "<real_type>"},{"type": "null"}] which falls under union type handling and then type Object would be generated loosing the actual type information from the schema. I think there is a possibility to do more on this front as from type generation perspective nullability is only a matter of type boxing. This PR is to address / recognise this exact scenario and use the type information from the union while leave other union type scenarios unchanged.

Changes

• added resolveNullableType method to top level SchemaRule to detect exactly this scenario and use boxed type
  • it gets all the remaining properties from the non-null node and copies to the original node and trigger the recursion, so they will be processed as usual

Edge case: when such a field is required then we should not add a NonNull / NotNull annotations since the field is nullable by default. To handle that I changed passing the property schema in type rules instead of the parent. It was only used at NotRequiredRule.java but there I could use parent instead.

The rest of the changes are testing related and some minor improvements. Any feedback is welcome!