Nullable Serializable Element
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.