gson.kt

GsonBuilder().registerTypeAdapterFactory(NullStringToEmptyAdapterFactory())
        .create()
        .run {
            return fromJson(this@toObject, T::class.java)
        }

class NullStringToEmptyAdapterFactory : TypeAdapterFactory {
    override fun <T> create(gson: Gson, type: TypeToken<T>): TypeAdapter<T>? {

        if (type.rawType != String::class.java) {
            return null
        }

        return StringAdapter() as TypeAdapter<T>
    }
}

class StringAdapter : TypeAdapter<String>() {
    override fun read(reader: JsonReader): String {
        if (reader.peek() == JsonToken.NULL) {
            reader.nextNull()

            return ""
        }

        return reader.nextString()
    }

    override fun write(writer: JsonWriter, value: String?) {
        if (value == null) {
            writer.nullValue()
        } else {
            writer.value(value)
        }
    }
}