Consider Unit response from type converter as 'not supported

[denis-zhdanov]

Problem

Currently it's possible to specify custom type converter via Context.Builder.withTypeConverter(). That function is expected to return a non-null value as an indication that it did a conversion and null otherwise.

So, a sample usage is like below:

builder.withTypeConverter { value, targetType ->
    if (targetType == Element::class) {
        Element(value.toString().toInt())
    } else {
        null
    }

However, it would be more convenient to treat the returned value of type Unit in the same way as null:

builder.withTypeConverter { value, targetType ->
    if (targetType == Element::class) {
        Element(value.toString().toInt())
    }