allow release-date and release-versions to be null

[mansona]

This allows empty release-dates and release-versions (which we can change when we have finished backfilling) and then once they are specified it will enforce the type 👍

i.e. this is how you implement "allows null". The "required" and "allowEmpty" are both quite buggy in revalidator so we have to do it a different way 👍

[kategengler]

Could you please open an issue here to restore it once they are in place?

I originally had the CI in emberjs/rfcs only fully linting changed files so that we could have the "real" rules for RFCs going through the process.

[mansona]

Done #6

Yes, I had to remove that past implementation because now that the RFCs need to be machine readable the all need to be equally machine readable. This will allow us to prevent the regression that happened when @wagenet added FIXME in places that should have been dates 👍