Gson: Treat null as empty string or empty array

MutableListTypeAdapter.kt

import com.google.gson.TypeAdapter
import com.google.gson.stream.JsonReader
import com.google.gson.stream.JsonToken
import com.google.gson.stream.JsonWriter
import java.io.IOException


class MutableListTypeAdapter<T>(private val delegate: TypeAdapter<MutableList<T>>?) : TypeAdapter<MutableList<T>>() {
    @Throws(IOException::class)
    override fun write(out: JsonWriter?, value: MutableList<T>?) {
        delegate?.write(out, value)
    }

    @Throws(IOException::class)
    override fun read(reader: JsonReader?): MutableList<T> {
        if (reader?.peek() === JsonToken.NULL) {
            reader.nextNull()
            return ArrayList()
        }
        return delegate?.read(reader) ?: ArrayList()
    }
}

NetModule.kt

// Imports

@Module
class NetModule() {    

    @Provides
    @Singleton
    fun provideGson(): Gson {
        return GsonBuilder()
                .registerTypeAdapterFactory(NullToEmptyAdapterFactory())
                .create()
    }
    
    // other provides
    // ...
}

NullToEmptyAdapterFactory.kt

import com.google.gson.Gson
import com.google.gson.TypeAdapter
import com.google.gson.TypeAdapterFactory
import com.google.gson.reflect.TypeToken


@Suppress("UNCHECKED_CAST")
internal class NullToEmptyAdapterFactory : TypeAdapterFactory {
    override fun <T> create(gson: Gson, type: TypeToken<T>): TypeAdapter<T>? {

        val delegate = gson.getDelegateAdapter(this, type)
        val rawType = type.rawType as? Class<T>
        return when (rawType) {
            String::class.java -> {
                StringTypeAdapter() as? TypeAdapter<T>
            }
            List::class.java -> {
                val mutableListDelegate = delegate as? TypeAdapter<MutableList<Any>>
                MutableListTypeAdapter(mutableListDelegate) as? TypeAdapter<T>
            }
            else -> {
                null
            }
        }
    }
}

StringTypeAdapter.kt

import com.google.gson.TypeAdapter
import com.google.gson.stream.JsonReader
import com.google.gson.stream.JsonToken
import com.google.gson.stream.JsonWriter
import java.io.IOException


class StringTypeAdapter : TypeAdapter<String>() {
    @Throws(IOException::class)
    override fun read(reader: JsonReader): String {
        if (reader.peek() === JsonToken.NULL) {
            reader.nextNull()
            return ""
        }
        return reader.nextString()
    }

    @Throws(IOException::class)
    override fun write(writer: JsonWriter, value: String?) {
        if (value == null) {
            writer.nullValue()
            return
        }
        writer.value(value)
    }
}