fpp-to-cpp: Autogen serialization hooks for empty structs

[brianrwillis]

Per the fpp documentation, empty structs are syntactically allowed. However, fpp-to-cpp creates neither operators nor serialization/deserialization functions for structs with no members. This is an issue for my team, as we have a number of commands from our GDS that have no parameters (e.g., NOOPs).
Our workaround is to create a padding byte with a default value, but this wastes bandwidth and clutters both our code and user-facing command GUI.

Thoughts?