Retrofit在不可为空的字符串(Kotlin)上返回null

class NullStringConverterFactory : Converter.Factory() {
    override fun responseBodyConverter(
        type: Type,
        annotations: Array<Annotation>,
        retrofit: Retrofit
    ): Converter<ResponseBody, *>? {
        val delegate = retrofit.nextResponseBodyConverter<Any>(this, type, annotations)
        return Converter<ResponseBody, Any> { body ->
            if (body.contentLength() == 0L) {
                null
            } else {
                delegate.convert(body)
            }
        }
    }
}

val retrofit = Retrofit.Builder()
    .baseUrl(BASE_URL)
    .addConverterFactory(NullStringConverterFactory())
    .build()