Fix json generation bug

If a message consists entirely of optional fields and none of them are set then the message gets serialized to Json as `{` instead of `{}`.
I have just checked this.

I did (on linux)

    rm ScalaBuff/ -rf
    git clone https://github.com/SandroGrzicic/ScalaBuff
    cd ScalaBuff/
    sbt test
    git diff

And saw similar changes. If these files are autogenerated as part of the build or test process, perhaps they should be ignored?