Clarify that fixity value must be non-empty JSON object

[zimeon]

Fixes #558

[pwinckles]

If you insist with this change, see the issue for arguments against it, I would suggest that you also add a general statement to the inventory section that specifies your expectation that optional keys do not appear in the json with null values. Statically typed json libraries will commonly serialize null values by default.

[zimeon]

I'm going to close this. It is clear from continued discussion that we don't want to forbid both null and {}