Make null message data optional

[apetro]

For all data in messages.json, where data is permissibly null, make the data outright optional so that omitting it has the same effect as if it were explicitly included and null (or whatever sensible default).

Thereby allowing convention over configuration: rather than explicitly saying a bunch of defaults about a message, don't have to configure these fields, just accept default convention.

Thereby making the data that we do configure more meaningful, since what's stated about any given message would only be what's non-default, interesting about it.