RPC definition should implicitly define input/output

Based on the discussion at https://mailarchive.ietf.org/arch/msg/netmod/tjxTtCehVLeSScoV84ggmvpNWb0, an RPC declaration should result in input and output statement being implicitly present even if they are not declared.

Hence the augmentation in the thread should succeed, just as if the rpc was defined with an empty input statement.