override fun <T : Any> decodeNullableSerializableElement(    descriptor: SerialDescriptor,     index: Int,     deserializer: DeserializationStrategy<T?>,     previousValue: T?): T?

Decodes nullable value of the type T with the given deserializer.

If value at given index was already decoded with previous decodeSerializableElement call with the same index, previousValue would contain a previously decoded value. This parameter can be used to aggregate multiple values of the given property to the only one. Implementation can safely ignore it and return a new value, efficiently using 'the last one wins' strategy, or apply format-specific aggregating strategies, e.g. appending scattered Protobuf lists to a single one.