[protobuf] null/nodata entries inside repeated collection

I am considering moving over from JSON to Protocol Buffers for my data intensive applications. However, one thing that I really need and can't find information about is the feature of 'null' or 'nodata' inside a repeated field. Suppose I have a large vector with booleans with observations, how

[protobuf] validating protobuf messages

I am using the RProtoBuf package, which interfaces to the c++ protobuf library. I was wondering if there is any way of 'validating' a pb message, in the sense that you can check if it actually is a valid message for a given pb description. Currently, RProtoBuf is very permissive and will basical