Add `JsonTypeInfo.As.NOTHING` value

[cowtowncoder]

So: there is already JsonTypeInfo.Id.NONE, but nothing matching for JsonTypeInfo.As to support "no type info actually added" case. Let's fix this (minor) gap.

[cowtowncoder]

cc @JacksonJang I think it makes sense to add this Enum value after all -- can be useful.

[JacksonJang]

Thank you :)