Add null value handling for kotlin TypesAdapters

[mikeyshean]

PR for #109

[djensen47]

@macisamuele @martinbonnin @cortinico Any thoughts on this PR?

We spent a day and a half debugging this issue and @mikeyshean discovered that toJson needs to handle null values otherwise it triggers an issue in Moshi thinking there is a nesting problem and exception gets thrown by Moshi.

[macisamuele]

@mikeyshean thank a lot for the contribution 🎉

The pr looks sane to me, but I need some time to verify it.
It looks surprising that we haven't face this issue before.

Something that for sure I'll ask is to have testing coverage over the change and ideally a unit test in samples/junit-test that ensures no further regressions around this.

[macisamuele]

As mentioned in the comment before we do need to have testing coverage over this change as a regression on this might lead again to the same bug.

I tried to play around to re-create a case similar to the one in #109 but I was not successful.

[macisamuele]

I would rather suggest to have concrete implementations dealing only with the "not-nullable" case and having XNullableJsonAdapter taking care of the null handling.

Something like

internal abstract class XNullableJsonAdapter<T> : JsonAdapter<T>() { 
    abstract fun fromNonNullString(nextString: String): T
    abstract fun toJsonNonNull(writer: JsonWriter, value: T)
    ...
}

This allows to focus the attention of nullable in the class related to nullable and to the T type encoding in the related adapter class

[klaszlo8207]

Any news on this?

[djensen47]

Sorry, the project we were using this on was canceled. @klaszlo8207 feel free to pick up this PR and add a regression test to it.