Empty type list

Not sure if this applies here, but from the Cap'n Proto docs:

> List amplification: A list of Void values or zero-size structs can have a very large element count while taking constant space on the wire. If the receiving application expects a list of structs, it will see these zero-sized elements as valid structs set to their default values. If it iterates through the list processing each element, it could spend a large amount of CPU time or other resources despite the message being small. To defend against this, the “traversal limit” should count a list of zero-sized elements as if each element were one word instead. This rule was introduced in the C++ implementation in commit 1048706.