Add support for Nullable primary data

Now you can have a complete null primary data, like in this example:

@JsonApiX("emptyData", isNullable = true)
@Serializable
class EmptyData : JsonApiModel()

@Serializable
@JsonApiXMeta("emptyData")
data class EmptyDataMeta(val owner: String) : Meta

In this case the EmptyData will act only as wrapper and contains the Meta object or in some cases the Errors object

It also supports have a nullable data class that might be null or not like in this example

@JsonApiX("emptyData", isNullable = true)
@Serializable
data class Student(
    val name: String = ""
) : JsonApiModel()

In this case all the data fields should have a default value, And I have wrote a lint rule to check this

This how I achieved this:

In the generated JsonApiX_ I made the data field to be nullable
In case the data is null the type is going to be null, which will stop the JsonApiDiscriminator from working, So used the TypeExtractor to guess the type from the parent data class

infinum / kotlin-jsonapix

Add support for Nullable primary data #51